config: remove unrequired AC_HEADER_STDC

Autoconf says:
"This macro is obsolescent, as current systems have conforming
header files. New programs need not use this macro".
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>

configure.ac

@@ -80,8 +80,6 @@ fi
 
 # Checks for libraries.
 
-# Checks for header files.
-AC_HEADER_STDC
 
 AC_SUBST([XORG_CFLAGS])
 AC_SUBST([moduledir])