glthread/*, strsignal: Support for MSVC.

glthread/*, strsignal: Support for MSVC.

* lib/glthread/cond.h: Define WIN32_LEAN_AND_MEAN, so as to avoid
including <winsock.h> on MSVC 9.
* lib/glthread/lock.h: Likewise.
* lib/glthread/thread.h: Likewise.
* lib/glthread/tls.h: Likewise.
* lib/glthread/yield.h: Likewise.
* lib/strsignal.c: Include <string.h> first. Don't include <unistd.h>
if HAVE_UNISTD_H is false.
* m4/strsignal.m4 (gl_PREREQ_STRSIGNAL): Test for <unistd.h>.