build: 'declaration-after-statement' is an error

not allowed in Windows VS, so make it also not allowed with GCC

Thanks to Martin,
https://github.com/OpenSC/OpenSC/pull/156#issuecomment-17538586

configure.ac

@@ -601,6 +601,8 @@ if test "$GCC" = "yes"; then
 	CFLAGS="-fno-strict-aliasing ${CFLAGS}"
 fi
 
+CFLAGS="${CFLAGS} -Werror=declaration-after-statement"
+
 AC_CONFIG_FILES([
 	Makefile
 	doc/Makefile