Don't require OpenGL when building without GL

CMakeLists.txt

@@ -213,13 +213,15 @@ if(BUILD_STATIC)
 endif()
 
 # Check dependencies
-if(NOT TARGET_GLES OR TARGET_DESKTOP_GLES)
-    set(OpenGL_GL_PREFERENCE GLVND) # since CMake 3.11
-    find_package(OpenGL REQUIRED)
-elseif(TARGET_GLES2)
-    find_package(OpenGLES2 REQUIRED)
-else()
-    find_package(OpenGLES3 REQUIRED)
+if(WITH_GL OR TARGET_GL)
+    if(NOT TARGET_GLES OR TARGET_DESKTOP_GLES)
+        set(OpenGL_GL_PREFERENCE GLVND) # since CMake 3.11
+        find_package(OpenGL REQUIRED)
+    elseif(TARGET_GLES2)
+        find_package(OpenGLES2 REQUIRED)
+    else()
+        find_package(OpenGLES3 REQUIRED)
+    endif()
 endif()
 
 # Configuration variables (saved later to configure.h)