Set HAVE_GRADLE in a more autoconfy way

It is perhaps more autoconfy to have AM_CONDITIONAL evaluate a variable
rather than call it with what are essentially constants representing
either true or false. It might be even more autoconfy to use
AC_CHECK_PROG here instead; I don't know if there's a good reason why
that wasn't already done here.

configure.ac

@@ -83,12 +83,11 @@ dnl ================================================================
 AC_MSG_CHECKING([for gradle])
 if command -v "$GRADLE" >/dev/null; then
     AC_MSG_RESULT([$GRADLE])
-    AM_CONDITIONAL(HAVE_GRADLE, [test 1 = 1])
 else
     GRADLE=
     AC_MSG_RESULT([no])
-    AM_CONDITIONAL(HAVE_GRADLE, [test 1 = 0])
 fi
+AM_CONDITIONAL(HAVE_GRADLE, [test x"$GRADLE" != x])
 
 AC_SUBST(GRADLE)