[wp][config] check presence of why3 _before_ checking its version number

it's more logical that way.

src/plugins/wp/configure.ac

@@ -56,6 +56,8 @@ REQUIRE_WHY3=
 USE_WHY3=
 HAS_WHY3=
 
+configure_pkg(why3,[package why3 not found])
+
 AC_MSG_CHECKING(why3 version)
 WHY3VERSION=`ocamlfind query -format %v why3 | tr -d '\\r\\n'`
 case $WHY3VERSION in
@@ -71,8 +73,6 @@ case $WHY3VERSION in
         ;;
 esac
 
-configure_pkg(why3,[package why3 not found])
-
 plugin_require_external(wp,why3)
 
 AC_SUBST(HAS_WHY3)