Try to allow including sys/stat.h by undefining _POSIX_TIMERS

We hoped that would skip parts of an include file which lead to
"error: #error unimplemented for this target". Somehow doesn't, though.

src/config.h

@@ -41,6 +41,10 @@
 /* enable strdup() declaration in string.h: */
 #define __MISC_VISIBLE 1
 
+#ifdef _POSIX_TIMERS
+#undef _POSIX_TIMERS
+#endif
+
 #ifndef _POSIX_SOURCE
 #define _POSIX_SOURCE
 #endif