Remove obsolescent AC_HEADER_STDC macro

We only used the resulting define in one place, and really,
these headers just have to be around or its not worth trying.

configure.ac

@@ -561,9 +561,6 @@ AC_TRY_COMPILE([#include <dirent.h>], [DIR *dir;],
     CFLAGS=$glib_save_CFLAGS
     AC_MSG_WARN([Could not determine POSIX flag. (-posix didn't work.)])))
 
-# Checks for header files.
-AC_HEADER_STDC
-
 # Checks for library functions.
 AC_FUNC_VPRINTF
 AC_FUNC_ALLOCA

glib/tests/hash.c

@@ -32,11 +32,9 @@
 #  include <config.h>
 #endif
 
-#if STDC_HEADERS
 #include <stdio.h>
 #include <string.h>
 #include <stdlib.h>
-#endif
 
 #include <glib.h>