GLEW: Define static + enabled and includes via SCons

This allows us not to have to hack our definitions in the upstream files,
making it easier to upgrade to newer versions in the future.

For the include paths to work, the headers are moved to a GL subfolder to
match their upstream location.

platform/haiku/detect.py

@@ -23,7 +23,8 @@ def get_opts():
 
 def get_flags():
 	return [
-		('builtin_zlib', 'no')
+		('builtin_zlib', 'no'),
+		#('glew', 'yes'), # TODO: investigate the GLEW situation on Haiku
 	]
 
 def configure(env):
@@ -52,7 +53,6 @@ def configure(env):
 
 	#env.Append(CCFLAGS=['-DFREETYPE_ENABLED'])
 	env.Append(CPPFLAGS = ['-DPTHREAD_NO_RENAME']) # TODO: enable when we have pthread_setname_np
-	#env.Append(CPPFLAGS = ['-DGLEW_ENABLED']) # TODO: investigate the GLEW situation on Haiku
 	env.Append(CPPFLAGS = ['-DOPENGL_ENABLED', '-DMEDIA_KIT_ENABLED'])
 	env.Append(CPPFLAGS = ['-DUNIX_ENABLED', '-DGLES2_ENABLED', '-DGLES_OVER_GL'])
 	env.Append(LIBS = ['be', 'game', 'media', 'network', 'bnetapi', 'z', 'GL'])