configure.ac: use AC_PROG_CC_C99
Various parts of Xfce, and libraries used by Xfce, already use C99 features. Therefore it is reasonable to expect the availibility of a capable compiler. Current GCC releases silently default to newer C standards, starting with gcc-5 which defaults to C11.
Replace AC_PROG_CC with AC_PROG_CC_C99 to switch the discovered, or specified, compiler to C99.
Signed-off-by: Olaf Hering firstname.lastname@example.org