PyAPI: re-enable the "bgl" module for headless builds

Instead of removing the `bgl` module, set all it's functions to stubs
so importing `bgl` or any of it's members doesn't raise an error.

This avoids problems for scripts that import bgl but don't call it's
functions when running in background mode.

source/blender/python/generic/CMakeLists.txt

@@ -17,6 +17,7 @@ set(INC_SYS
 )
 
 set(SRC
+  bgl.c
   bl_math_py_api.c
   blf_py_api.c
   bpy_threads.c
@@ -26,6 +27,7 @@ set(SRC
   py_capi_rna.c
   py_capi_utils.c
 
+  bgl.h
   bl_math_py_api.h
   blf_py_api.h
   idprop_py_api.h
@@ -38,14 +40,6 @@ set(SRC
   python_utildefines.h
 )
 
-if(WITH_OPENGL)
-  list(APPEND SRC
-    bgl.c
-
-    bgl.h
-  )
-endif()
-
 set(LIB
   ${GLEW_LIBRARY}
   ${PYTHON_LINKFLAGS}