configure.ac: Use -std=c90 and error on declaration-after-statement

Replace gcc option of -std=c9x with c90 and add
-Werror=declaration-after-statement
This ensures we keep compatibility with msvc builds.

configure.ac

@@ -254,12 +254,13 @@ if test "x$GCC" = "xyes"; then
   JH_ADD_CFLAG([-Wall])
   JH_ADD_CFLAG([-Werror=unused-variable])
   JH_ADD_CFLAG([-fno-strict-aliasing])
+  JH_ADD_CFLAG([-Werror=declaration-after-statement])
 
   case $host_os in
   solaris*)
     ;;
   *)
-    JH_ADD_CFLAG([-std=c9x])
+    JH_ADD_CFLAG([-std=c90])
     ;;
   esac