mesa: Ensure gl* symbols are marked as dllexport on windows.

src/mesa/SConscript

@@ -12,6 +12,12 @@ if env['platform'] != 'winddk':
 		'#/src/mesa',
 	])
 	
+	if env['platform'] == 'windows':
+		env.Append(CPPDEFINES = [
+			'_GDI32_', # prevent gl* being declared __declspec(dllimport) in MS headers
+			'BUILD_GL32', # declare gl* as __declspec(dllexport) in Mesa headers 
+		])	 
+
 	#
 	# Source files
 	#