compat: add a basename() compatibility function

Some systems such as Windows lack libgen.h so provide a
basename() implementation for cross-platform use.

This introduces the NO_LIBGEN_H construct to the Makefile
and autoconf scripts.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -97,6 +97,13 @@
 #include "compat/mingw.h"
 #endif	/* __MINGW32__ */
 
+#ifndef NO_LIBGEN_H
+#include <libgen.h>
+#else
+#define basename gitbasename
+extern char *gitbasename(char *);
+#endif
+
 #ifndef NO_ICONV
 #include <iconv.h>
 #endif