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

@@ -1,27 +1,29
 /*
  * Utility functions
  *
  * Copyright (c) 2011 Yuya Nishihara <[e-mail]>
  *
  * This software may be used and distributed according to the terms of the
  * GNU General Public License version 2 or any later version.
  */
 
 #ifndef UTIL_H_
 #define UTIL_H_
 
 #ifdef __GNUC__
 #define PRINTF_FORMAT_ __attribute__((format(printf, 1, 2)))
+#else
+#define PRINTF_FORMAT_
 #endif
 
 void abortmsg(const char *fmt, ...) PRINTF_FORMAT_;
 
 void enabledebugmsg(void);
 void debugmsg(const char *fmt, ...) PRINTF_FORMAT_;
 
 void *mallocx(size_t size);
 void *reallocx(void *ptr, size_t size);
 
 int runshellcmd(const char *cmd, const char *envp[], const char *cwd);
 
 #endif  /* UTIL_H_ */