configure.ac: AC_PROG_CC->AC_PROG_CC_STDC

This addresses a failure I ran into on el7 where we failed to build
because -std=c99 wasn't utilized during compile.

Signed-off-by: Tony Asleson <tasleson@redhat.com>

configure.ac

@@ -46,7 +46,7 @@ LIBLED_LIBTOOL_VERSION=$CURRENT:$REVISION:$AGE
 
 AC_SUBST([LIBLED_LIBTOOL_VERSION])
 
-AC_PROG_CC
+AC_PROG_CC_STDC
 AC_PROG_INSTALL
 
 AC_CONFIG_HEADERS([config_ac.h])