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.

(cherry picked from commit 36738ddda4)

platform/windows/detect.py

@@ -168,7 +168,6 @@ def get_opts():
 def get_flags():
 
 	return [
-		('glew','yes'),
 		('openssl','builtin'), #use builtin openssl
 	]