config: remove AC_PROG_CC as it overrides AC_PROG_C_C99

XORG_STRICT_OPTION from XORG_DEFAULT_OPTIONS calls
AC_PROG_C_C99. This sets gcc with -std=gnu99.
If AC_PROG_CC macro is called afterwards, it resets CC to gcc.
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>

configure.ac

@@ -43,7 +43,6 @@ XORG_DEFAULT_OPTIONS
 # Checks for programs.
 AC_DISABLE_STATIC
 AC_PROG_LIBTOOL
-AC_PROG_CC
 
 AH_TOP([#include "xorg-server.h"])