test with cmake 3.8 since that's the min required cmake version for dlib (#2748)

* test with cmake 3.8 since that's the min required cmake version for dlib

.github/workflows/build_cpp.yml

@@ -10,7 +10,6 @@ on:
 
 env:
   config: Release
-  build_dir: build
 
 defaults:
   run:
@@ -18,22 +17,42 @@ defaults:
     working-directory: dlib/test
 
 jobs:
-  ubuntu-latest-gcc-default:
+  ubuntu-latest-gcc-default-cmake-3-8:
     runs-on: 'ubuntu-latest'
     steps:
     - uses: actions/checkout@v2
+
+    - uses: actions/cache@v3
+      id: cache-cmake-download
+      with:
+        # cache this folder:
+        path: ~/cmake-3.8.0-Linux-x86_64
+        key: cmake-3.8.0_try3
+    - run: |
+        # Get the minimum version of cmake dlib supports
+        wget https://cmake.org/files/v3.8/cmake-3.8.0-Linux-x86_64.tar.gz
+        tar -xf cmake-3.8.0-Linux-x86_64.tar.gz -C ~
+      if: steps.cache-cmake-download.outputs.cache-hit != 'true'
+
     - name: Install dependencies
       run: |
         sudo apt update
-        sudo apt install libwebp-dev libavformat-dev libavcodec-dev libavdevice-dev libavfilter-dev libswresample-dev libswscale-dev libavutil-dev 
+        sudo apt install libwebp-dev libavformat-dev libavcodec-dev libavdevice-dev libavfilter-dev libswresample-dev libswscale-dev libavutil-dev make
     - name: Configure
-      run: cmake ${{ github.workspace }}/dlib/test -B ${{ env.build_dir }}
+      run: |
+        mkdir build
+        cd build
+        ~/cmake-3.8.0-Linux-x86_64/bin/cmake .. 
     - name: Build just tests
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --target dtest --parallel 4
+      run: |
+        cd build
+        make -j4 dtest
     - name: Test
-      run: ${{ env.build_dir }}/dtest --runall -q
+      run: build/dtest --runall -q
     - name: Build examples, etc
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --parallel 2
+      run: |
+        cd build
+        make -j2
 
   ubuntu-latest-gcc-11:
     runs-on: 'ubuntu-latest'
@@ -48,13 +67,13 @@ jobs:
         sudo apt install gcc-11 g++-11
         sudo update-alternatives --install /usr/bin/gcc gcc /usr/bin/gcc-11 110 --slave /usr/bin/g++ g++ /usr/bin/g++-11 --slave /usr/bin/gcov gcov /usr/bin/gcov-11
     - name: Configure
-      run: cmake ${{ github.workspace }}/dlib/test -B ${{ env.build_dir }}
+      run: cmake ${{ github.workspace }}/dlib/test -B build
     - name: Build just tests
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --target dtest --parallel 4
+      run: cmake --build build --config Release --target dtest --parallel 4
     - name: Test
-      run: ${{ env.build_dir }}/dtest --runall -q
+      run: build/dtest --runall -q
     - name: Build examples, etc
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --parallel 2
+      run: cmake --build build --config Release --parallel 2
 
   ubuntu-18_04-gcc-7:
     runs-on: 'ubuntu-18.04'
@@ -68,13 +87,13 @@ jobs:
       run: |
         sudo apt install gcc-7 g++-7
     - name: Configure
-      run: cmake ${{ github.workspace }}/dlib/test -B ${{ env.build_dir }}
+      run: cmake ${{ github.workspace }}/dlib/test -B build
     - name: Build just tests
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --target dtest --parallel 4
+      run: cmake --build build --config Release --target dtest --parallel 4
     - name: Test
-      run: ${{ env.build_dir }}/dtest --runall -q
+      run: build/dtest --runall -q
     - name: Build examples, etc
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --parallel 2
+      run: cmake --build build --config Release --parallel 2
 
   ubuntu-latest-clang-default:
     runs-on: 'ubuntu-latest'
@@ -88,13 +107,13 @@ jobs:
       run: |
         export CC=/usr/bin/clang
         export CXX=/usr/bin/clang++
-        cmake ${{ github.workspace }}/dlib/test -B ${{ env.build_dir }}
+        cmake ${{ github.workspace }}/dlib/test -B build
     - name: Build just tests
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --target dtest --parallel 4
+      run: cmake --build build --config Release --target dtest --parallel 4
     - name: Test
-      run: ${{ env.build_dir }}/dtest --runall -q
+      run: build/dtest --runall -q
     - name: Build examples, etc
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --parallel 2
+      run: cmake --build build --config Release --parallel 2
 
   ubuntu-latest-clang-13:
     runs-on: 'ubuntu-latest'
@@ -113,13 +132,13 @@ jobs:
       run: |
         export CC=/usr/bin/clang-13
         export CXX=/usr/bin/clang++-13
-        cmake ${{ github.workspace }}/dlib/test -B ${{ env.build_dir }}
+        cmake ${{ github.workspace }}/dlib/test -B build
     - name: Build just tests
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --target dtest --parallel 4
+      run: cmake --build build --config Release --target dtest --parallel 4
     - name: Test
-      run: ${{ env.build_dir }}/dtest --runall -q
+      run: build/dtest --runall -q
     - name: Build examples, etc
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --parallel 2
+      run: cmake --build build --config Release --parallel 2
 
   windows-latest:
     runs-on: 'windows-latest'
@@ -129,13 +148,13 @@ jobs:
       run: |
         # don't use CMake 3.25.0 https://gitlab.kitware.com/cmake/cmake/-/issues/23975
         pip3 install cmake==3.24.0
-        cmake . -B ${{ env.build_dir }}
+        cmake . -B build
     - name: Build just tests
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --target dtest --parallel 4
+      run: cmake --build build --config Release --target dtest --parallel 4
     - name: Test
-      run: ${{ env.build_dir }}/${{ env.config }}/dtest.exe --runall -q
+      run: build/Release/dtest.exe --runall -q
     - name: Build ancillary tools
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --target imglab htmlify dtoc --parallel 4
+      run: cmake --build build --config Release --target imglab htmlify dtoc --parallel 4
 
   windows-2019:
     runs-on: 'windows-2019'
@@ -145,11 +164,11 @@ jobs:
       run: |
         # don't use CMake 3.25.0 https://gitlab.kitware.com/cmake/cmake/-/issues/23975
         pip3 install cmake==3.24.0
-        cmake . -B ${{ env.build_dir }}
+        cmake . -B build
     - name: Build just tests
-      run: cmake --build ${{ env.build_dir }} --config Debug --target dtest --parallel 4
+      run: cmake --build build --config Debug --target dtest --parallel 4
     - name: Build ancillary tools
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --target imglab htmlify dtoc --parallel 4
+      run: cmake --build build --config Release --target imglab htmlify dtoc --parallel 4
 
   macos-latest:
     runs-on: 'macos-latest'
@@ -157,10 +176,10 @@ jobs:
     - uses: actions/checkout@v2
     - name: Configure
       # MacOS machines often come with low quality BLAS libraries installed, so don't use those.
-      run: cmake ${{ github.workspace }}/dlib/test -B ${{ env.build_dir }} -DDLIB_USE_BLAS=0 -DDLIB_USE_LAPACK=0
+      run: cmake ${{ github.workspace }}/dlib/test -B build -DDLIB_USE_BLAS=0 -DDLIB_USE_LAPACK=0
     - name: Build just tests
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --target dtest --parallel 4
+      run: cmake --build build --config Release --target dtest --parallel 4
     - name: Test
-      run: ${{ env.build_dir }}/dtest --runall --no_test_timer -q
+      run: build/dtest --runall --no_test_timer -q
     - name: Build examples, etc
-      run: cmake --build ${{ env.build_dir }} --config ${{ env.config }} --parallel 2
+      run: cmake --build build --config Release --parallel 2