build: make sure we check for c99 during configure

We have some c99 code in expedite (loop initial declarations) and some
setups bail out when trying to use c89 here. EFL is already enforcing
c99 and compilers have catched up a long time ago.

Fixes T5440

configure.ac

@@ -69,9 +69,14 @@ EFL_ENABLE_BETA_API_SUPPORT
 
 AC_HEADER_STDC
 AC_C_CONST
+AC_PROG_CC_C99
 AM_PROG_CC_C_O
 AC_C___ATTRIBUTE__
 
+if test "x${ac_cv_prog_cc_c99}" = "xno" ; then
+   AC_MSG_ERROR([expedite requires a c99-capable compiler])
+fi
+
 WIN32_CFLAGS=""
 case "$host_os" in
    cegcc*)