Fix CMake projects, restore testing with OpenGL disabled

.travis.yml

@@ -35,6 +35,10 @@ matrix:
           env: OPENRCT2_CMAKE_OPTS="-DDISABLE_NETWORK=ON -DDISABLE_HTTP_TWITCH=ON" TARGET=docker32
           services:
             - docker
+        - os: linux
+          env: OPENRCT2_CMAKE_OPTS="-DDISABLE_OPENGL=ON" TARGET=docker32
+          services:
+            - docker
         - os: osx
           osx_image: xcode7.3
           env:

CMakeLists.txt

@@ -58,6 +58,9 @@ endif ()
 
 if (DISABLE_OPENGL)
 	add_definitions(-DDISABLE_OPENGL)
+else (DISABLE_OPENGL)
+	# Makes OpenGL function get queried in run-time rather than linked-in
+	add_definitions(-DOPENGL_NO_LINK)
 endif (DISABLE_OPENGL)
 
 if (DISABLE_NETWORK)