Don't require OpenGL when building without GL

CMakeLists.txt

@@ -213,6 +213,7 @@ if(BUILD_STATIC)
 endif()
 
 # Check dependencies
+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)
@@ -221,6 +222,7 @@ elseif(TARGET_GLES2)
     else()
         find_package(OpenGLES3 REQUIRED)
     endif()
+endif()
 
 # Configuration variables (saved later to configure.h)
 if(TARGET_GL)