glew: Split thirdparty files and isolate env

Not fully happy about the way this one interacts with the various
platforms. Maybe the platform_config.h should be generated by the
SCsub instead of passing a define just to know where is the header.

drivers/gl_context/SCsub

@@ -1,11 +1,26 @@
 Import('env')
+Import('env_drivers')
 
-env.add_source_files(env.drivers_sources,"*.cpp")
+env_glew = env_drivers.Clone()
+
+if (env["platform"] in ["haiku","osx","windows","x11"]):
+	# Thirdparty source files
+	if (env["glew"] != "system"): # builtin
+		thirdparty_dir = "#thirdparty/glew/"
+		thirdparty_sources = [
+			"glew.c",
+		]
+		thirdparty_sources = [thirdparty_dir + file for file in thirdparty_sources]
+
+		env_glew.add_source_files(env.drivers_sources, thirdparty_sources)
+		env_glew.Append(CPPFLAGS = ['-DGLEW_STATIC'])
+		env_glew.Append(CPPPATH = [thirdparty_dir])
+		# Also pass to global env, used by platform code
+		env.Append(CPPPATH = [thirdparty_dir])
 
-if (env.get('glew') == 'yes'):
-	env.add_source_files(env.drivers_sources,"glew.c")
 	env.Append(CPPFLAGS = ['-DGLEW_ENABLED'])
-	env.Append(CPPFLAGS = ['-DGLEW_STATIC'])
-	env.Append(CPPPATH = ['.'])
+
+# Godot source files
+env_glew.add_source_files(env.drivers_sources, "*.cpp")
 
 Export('env')