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 <olaf@aepfle.de>

configure.ac.in

@@ -69,7 +69,7 @@ fi
 dnl ********************************
 dnl *** Check for basic programs ***
 dnl ********************************
-AC_PROG_CC()
+AC_PROG_CC_C99
 AC_PROG_LD()
 AM_PROG_CC_C_O()
 AC_PROG_INSTALL()