autotools: support using a custom gradle command

Makefile.am

 
 clean-local:
 	cd src && rm -f *.o *~ && cd ..
-	cd android && gradle clean
+	cd android && $(GRADLE) clean
 
 distclean-local:
 	rm -rf espeak-data/phondata-manifest
 	cd android && ndk-build
 
 apk-release:
-	cd android && gradle assembleRelease
+	cd android && $(GRADLE) assembleRelease
 
 apk-debug:
-	cd android && gradle assembleDebug
+	cd android && $(GRADLE) assembleDebug
 
 apk-check:
-	cd android && gradle connectedCheck
+	cd android && $(GRADLE) connectedCheck
 
 android/res/raw/espeakdata.zip: espeak-data/phontab dictionaries
 	mkdir -pv android/res/raw

configure.ac

     AC_MSG_ERROR([Please ensure that the Android NDK is installed and ndk-build is usable from the command line.])
 fi
 
+dnl ================================================================
+dnl gradle checks.
+dnl ================================================================
+
+AC_ARG_WITH([gradle],
+    [AS_HELP_STRING([--with-gradle], [specify the gradle program to use @<:@default=gradle@:>@])],
+    [GRADLE=$with_gradle],
+    [GRADLE=gradle])
+
+AC_MSG_CHECKING([for gradle])
+if test -e ${GRADLE} ; then
+    AC_MSG_RESULT([${GRADLE}])
+else
+    AC_MSG_RESULT([no])
+    AC_MSG_ERROR([The gradle command '${GRADLE}' is not found.])
+fi
+
+AC_SUBST(GRADLE)
+
 dnl ================================================================
 dnl getopt checks.
 dnl ================================================================
         sada:                          ${have_sada}
         audio configuration:           ${AUDIO}
 
+        gradle:                        ${GRADLE}
+
         Klatt:                         ${have_klatt}
         MBROLA:                        ${have_mbrola}
         Sonic:                         ${have_sonic}