ref_gl: add an option to disable ref_gl building

ref_gl/wscript

@@ -19,6 +19,9 @@ def options(opt):
 	grp.add_option('--enable-gles2', action='store_true', dest='GLWES',
 	    help = 'enable gles2 renderer by linking gl-wes-v2 statically(put source to ref_gl directory)')
 	
+	grp.add_option('--disable-gl', action='store_false', dest='GL', default=True,
+		help = 'disable building OpenGL renderer(default: False)')
+
 	# stub
 	return
 
@@ -66,6 +69,7 @@ def build(bld):
 		if glstatic: return ['XASH_GL_STATIC']
 		else: return []
 
+	if bld.env.GL:
 		bld.shlib(
 			source   = source,
 			target   = 'ref_gl',