chg: define PRINTF_FORMAT_ for non gnu C compiler

Before this patch, if __GNUC__ is not defined, PRINTF_FORMAT_ will not be
defined and will cause compilation error.

This patch solves the issue by making sure PRINTF_FORMAT_ is defined. It
allows chg to be compiled with tcc (http://bellard.org/tcc/) by:

  tcc -o chg *.c

contrib/chg/util.h

@@ -12,6 +12,8 b''
 
 #ifdef __GNUC__
 #define PRINTF_FORMAT_ __attribute__((format(printf, 1, 2)))
+#else
+#define PRINTF_FORMAT_
 #endif
 
 void abortmsg(const char *fmt, ...) PRINTF_FORMAT_;