compat: add a mkstemps() compatibility function

mkstemps() is a BSD extension so provide an implementation
for cross-platform use.

Signed-off-by: David Aguilar <davvid@gmail.com>
Tested-by: Johannes Sixt <j6t@kdbg.org> (Windows)
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -232,6 +232,11 @@ extern int gitsetenv(const char *, const char *, int);
 extern char *gitmkdtemp(char *);
 #endif
 
+#ifdef NO_MKSTEMPS
+#define mkstemps gitmkstemps
+extern int gitmkstemps(char *, int);
+#endif
+
 #ifdef NO_UNSETENV
 #define unsetenv gitunsetenv
 extern void gitunsetenv(const char *);