windows-tls: New module. * lib/windows-tls.h: New file, based on lib/glthread/tls.h. * lib/windows-tls.c: New file, based on lib/glthread/tls.h. * lib/glthread/tls.h: Include windows-tls.h. (gl_tls_key_t): Define using glwthread_tls_key_t. * modules/windows-tls: New file. * modules/tls (Depends-on): Add windows-tls.