Show in configure summary when gradle is not found

Previously, if configure determined that gradle could not be found, the
summary at the end of the configure output would nevertheless print the
name or path of the nonexistent gradle executable that it tried to find.
Now it clears that variable.

configure.ac

@@ -85,6 +85,7 @@ 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