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>

compat/basename.c

@@ -0,0 +1,15 @@
+#include "../git-compat-util.h"
+
+/* Adapted from libiberty's basename.c.  */
+char *gitbasename (char *path)
+{
+	const char *base;
+	/* Skip over the disk name in MSDOS pathnames. */
+	if (has_dos_drive_prefix(path))
+		path += 2;
+	for (base = path; *path; path++) {
+		if (is_dir_sep(*path))
+			base = path + 1;
+	}
+	return (char *)base;
+}