Allow the build to work without gradle (for non-Android builds).

configure.ac

     AC_MSG_RESULT([${GRADLE}])
 else
     AC_MSG_RESULT([no])
-    AC_MSG_ERROR([The gradle command '${GRADLE}' is not found.])
 fi
 
 AC_SUBST(GRADLE)
         Sonic:                         ${have_sonic}
         PCAudioLib:                    ${have_pcaudiolib}
 
-        gradle:                        ${GRADLE}
+        gradle (Android):              ${GRADLE}
 
         Klatt:                         ${have_klatt}
         MBROLA:                        ${have_mbrola}