Added travis tests to make sure we work with old versions of cmake.

.travis.yml

@@ -38,6 +38,15 @@ matrix:
       script:
         - dlib/travis/build-and-test.sh
 
+    ###################
+    - language: cpp
+      compiler: gcc
+      os: linux
+      env:
+        - VARIANT=old-cmake
+      script:
+        - dlib/travis/build-and-test.sh
+
     ###################
     - language: cpp
       compiler: gcc
@@ -104,4 +113,9 @@ matrix:
     #    - dlib/travis/build-and-test.sh
          
           
-
+ cache:
+   directories:
+     - cmake
+ 
+ before_install:
+   - dlib/travis/before-install.sh

dlib/travis/before-install.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+# Exit if anything fails.
+set -eux
+
+## download CMAKE to get colored output
+if [[ ! -x cmake/3.5/bin/cmake && -d cmake ]]; then
+    rm -rf cmake
+fi
+if [[ ! -d cmake ]]; then
+  CMAKE_URL="http://www.cmake.org/files/v3.5/cmake-3.5.2-Linux-x86_64.tar.gz"
+  mkdir -p cmake/3.5
+  wget --no-check-certificate -O - ${CMAKE_URL} | tar --strip-components=1 -xz -C cmake/3.5
+
+  CMAKE_URL="http://www.cmake.org/files/v2.8/cmake-2.8.12-Linux-i386.tar.gz"
+  mkdir -p cmake/2.8
+  wget --no-check-certificate -O - ${CMAKE_URL} | tar --strip-components=1 -xz -C cmake/2.8
+
+  CMAKE_URL="http://www.cmake.org/files/v3.1/cmake-3.1.2-Linux-x86_64.tar.gz"
+  mkdir -p cmake/3.1
+  wget --no-check-certificate -O - ${CMAKE_URL} | tar --strip-components=1 -xz -C cmake/3.1
+fi

dlib/travis/build-and-test.sh

@@ -30,6 +30,23 @@ if [ "$VARIANT" = "tools" ]; then
   cmake --build .  -- -j 2
 fi
 
+# The point of this test is just to make sure the cmake scripts work with the
+# oldest version of cmake we are supposed to support.
+if [ "$VARIANT" = "old-cmake" ]; then
+  mkdir build
+  cd build
+  ../cmake/2.8/bin/cmake ../dlib/test/tools 
+  ../cmake/2.8/bin/cmake --build .  -- -j 2
+
+  rm -rf *
+  ../cmake/3.1/bin/cmake ../dlib/test/tools 
+  ../cmake/3.1/bin/cmake --build .  -- -j 2
+
+  rm -rf *
+  ../cmake/3.5/bin/cmake ../dlib/test/tools 
+  ../cmake/3.5/bin/cmake --build .  -- -j 2
+fi
+
 if [ "$VARIANT" = "examples" ]; then
   mkdir build
   cd build