[client] opengl: make ds functions optional

Using a macro ENABLE_OPENGL just like ENABLE_EGL to optionally remove
OpenGL implementation code. This is mostly because on Wayland it's just
a rehash of the EGL code (as EGL is the only way to create OpenGL
contexts on Wayland).

client/CMakeLists.txt

@@ -53,6 +53,10 @@ add_compile_options(
 set(EXE_FLAGS "-Wl,--gc-sections -z noexecstack")
 set(CMAKE_C_STANDARD 11)
 
+if (ENABLE_OPENGL)
+  add_definitions(-D ENABLE_OPENGL)
+endif()
+
 if (ENABLE_EGL)
   add_definitions(-D ENABLE_EGL)
 endif()