- use environment variables

- on ubuntu 18 gcc7, use ffmpeg 3.2.18

.github/workflows/build_cpp.yml

@@ -16,6 +16,14 @@ defaults:
 jobs:
   ubuntu-latest-gcc-default-cmake-3-8-ffmpeg5:
     runs-on: 'ubuntu-latest'
+
+    env:
+      ffmpeg_tar:     n5.1.3.tar.gz
+      ffmpeg_url:     https://github.com/FFmpeg/FFmpeg/archive/refs/tags/n5.1.3.tar.gz
+      ffmpeg_src:     FFmpeg-n5.1.3
+      ffmpeg_install: /home/runner/ffmpeg-n5.1.3_installation
+      ffmpeg_key:     ffmpeg-n5.1.3_try4
+      
     steps:
     - uses: actions/checkout@v2
 
@@ -42,16 +50,16 @@ jobs:
       uses: actions/cache@v3
       id: cache-ffmpeg5
       with:
-        path: /home/runner/ffmpeg-n5.1.3_installation
-        key: ffmpeg-n5.1.3_try4
+        path: $ffmpeg_install
+        key: $ffmpeg_key
         
     - name: Build FFmpeg 5
       if: steps.cache-ffmpeg5.outputs.cache-hit != 'true'
       run: |
-        wget https://github.com/FFmpeg/FFmpeg/archive/refs/tags/n5.1.3.tar.gz
-        tar -xf n5.1.3.tar.gz
-        cd FFmpeg-n5.1.3
-        ./configure --prefix=/home/runner/ffmpeg-n5.1.3_installation --disable-doc --disable-programs
+        wget $ffmpeg_url
+        tar -xf $ffmpeg_tar
+        cd $ffmpeg_src
+        ./configure --prefix=$ffmpeg_install --disable-doc --disable-programs
         make -j4
         make install
         cd ..
@@ -60,7 +68,7 @@ jobs:
       run: |
         mkdir build
         cd build
-        ~/cmake-3.8.0-Linux-x86_64/bin/cmake -DCMAKE_PREFIX_PATH=/home/runner/ffmpeg-n5.1.3_installation .. 
+        ~/cmake-3.8.0-Linux-x86_64/bin/cmake -DCMAKE_PREFIX_PATH=$ffmpeg_install .. 
     
     - name: Build just tests
       run: |
@@ -107,27 +115,56 @@ jobs:
 
   ubuntu-18_04-gcc-7:
     runs-on: 'ubuntu-18.04'
+
+    env:
+      ffmpeg_tar:     n3.2.18.tar.gz
+      ffmpeg_url:     https://github.com/FFmpeg/FFmpeg/archive/refs/tags/n3.2.18.tar.gz
+      ffmpeg_src:     FFmpeg-n3.2.18
+      ffmpeg_install: /home/runner/ffmpeg-n3.2.18_installation
+      ffmpeg_key:     ffmpeg-n3.2.18_try1
+
     steps:
     - uses: actions/checkout@v2
+
     - 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 
-    - name: Install gcc 7
+        sudo apt install gcc-7 g++-7 make libwebp-dev 
+
+    - name: Cache FFmpeg 3.2
+      uses: actions/cache@v3
+      id: cache-ffmpeg3.2
+      with:
+        path: $ffmpeg_install
+        key: $ffmpeg_key
+        
+    - name: Build FFmpeg 3.2
+      if: steps.cache-ffmpeg3.2.outputs.cache-hit != 'true'
       run: |
-        sudo apt install gcc-7 g++-7
+        wget $ffmpeg_url
+        tar -xf $ffmpeg_tar
+        cd $ffmpeg_src
+        ./configure --prefix=$ffmpeg_install --disable-doc --disable-programs
+        make -j4
+        make install
+        cd ..
+
     - name: Configure
-      run: cmake ${{ github.workspace }}/dlib/test -B build
+      run: cmake ${{ github.workspace }}/dlib/test -B build -DCMAKE_PREFIX_PATH=$ffmpeg_install
+
     - name: Build just tests
       run: cmake --build build --config Release --target dtest --parallel 4
+
     - name: Test
       run: build/dtest --runall -q
+
     - name: Build examples, etc
       run: cmake --build build --config Release --parallel 2
 
     # Test cmake scrips can build standalone dlib as a shared library
     - name: Configure dlib as shared library  
-      run: cmake ${{ github.workspace }}/dlib -B build_dlib_shared -DBUILD_SHARED_LIBS=1
+      run: cmake ${{ github.workspace }}/dlib -B build_dlib_shared -DBUILD_SHARED_LIBS=1 -DCMAKE_PREFIX_PATH=$ffmpeg_install
+
     - name: Build dlib as shared library
       run: cmake --build build_dlib_shared --parallel 4