We now build GLEW always (unless a system one is used)

So its headers are always there in workdir.

Change-Id: I513b49da52c1f393c02b24c5bcf0f415e22e79d2
No glu.h in OpenGL ES for iOS

Change-Id: Ie0eb1f68d997c4f2bd37c9d7c2ff6feccbc7b8ce
Include <Availability.h> to get __IPHONE_OS_VERSION_MIN_REQUIRED

Change-Id: I0ef21ee7709cab05639935d70722973c9a18f93c
Make it compile for iOS and maybe Android, too

Don't include X11 headers where there is no X.

Include a copy of gluCheckExtension as there is no glu on either iOS nor Android.

Change-Id: Idf387a4eda5a8410d1511d6e0db78c0742af5a99