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>