Check more accurately if gradle exists

Previously, the configure script would only successfully determine that
gradle exists if the user specified an absolute path to the executable.
It would fail if the user specified only an executable name or if the
default value of "gradle" were used. Now it should succeed in either
situation.

configure.ac

@@ -81,7 +81,7 @@ dnl gradle checks.
 dnl ================================================================
 
 AC_MSG_CHECKING([for gradle])
-if test -e ${GRADLE} ; then
+if command -v "$GRADLE" >/dev/null; then
     AC_MSG_RESULT([${GRADLE}])
     AM_CONDITIONAL(HAVE_GRADLE, [test 1 = 1])
 else