Build GLEW for all platforms

Whether it actually should be used or not on some particular platform
is another thing that remains to be seen. But I think it should be
buildable for all?

Don't build the glew programs for non-desktop platforms.

Build only static libraries when DISABLE_DYNLOADING.

No AGL framework, and it's OpenGLES, not OpenGL, on iOS.

Change-Id: Idf3aad341c4f270952db2039d5a5e421f39e728f