Add a separate build option for the OpenGL renderer

src/win/Makefile.mingw

@@ -63,6 +63,9 @@ ifeq ($(DEV_BUILD), y)
  ifndef OPEN_AT
   OPEN_AT	:= y
  endif
+ ifndef OPENGL
+  OPENGL	:= y
+ endif
  ifndef PAS16
   PAS16		:= n
  endif
@@ -133,6 +136,9 @@ else
  ifndef OPEN_AT
   OPEN_AT	:= n
  endif
+ ifndef OPENGL
+  OPENGL	:= n
+ endif
  ifndef PAS16
   PAS16		:= n
  endif
@@ -433,7 +439,7 @@ ifeq ($(WX), y)
  UIOBJ		:= wx_main.o wx_ui.o wx_stbar.o wx_render.o
 else
  UIOBJ		:= win_ui.o win_stbar.o \
-		   win_sdl.o win_opengl.o win_opengl_glslp.o glad.o \
+		   win_sdl.o \
 		   win_dialog.o win_about.o \
 		   win_settings.o win_devconf.o win_snd_gain.o win_specify_dim.o \
 		   win_new_floppy.o win_jsconf.o win_media_menu.o
@@ -527,6 +533,12 @@ ifeq ($(OPEN_AT), y)
 OPTS		+= -DUSE_OPEN_AT
 endif
 
+ifeq ($(OPENGL), y)
+OPTS		+= -DUSE_OPENGL
+RFLAGS		+= -DUSE_OPENGL
+DEVBROBJ	+= win_opengl.o win_opengl_glslp.o glad.o
+endif
+
 ifeq ($(PAS16), y)
 OPTS		+= -DUSE_PAS16
 DEVBROBJ	+= snd_pas16.o