Fix running 'make clean' when gradle is not present. Gradle is used for the Android build and is not needed when just building eSpeak NG on Linux/BSD systems.

Makefile.am

@@ -59,7 +59,9 @@ install-exec-hook:
 clean-local:
 	cd src && rm -f *.o *~ && cd ..
 	cd src/libespeak-ng/ && rm -f *.o *.lo && cd ../..
+if HAVE_GRADLE
 	cd android && $(GRADLE) clean
+endif
 
 distclean-local:
 	rm -rf espeak-ng-data/phondata-manifest

configure.ac

@@ -33,8 +33,10 @@ AC_ARG_WITH([gradle],
 AC_MSG_CHECKING([for gradle])
 if test -e ${GRADLE} ; then
     AC_MSG_RESULT([${GRADLE}])
+    AM_CONDITIONAL(HAVE_GRADLE, [1])
 else
     AC_MSG_RESULT([no])
+    AM_CONDITIONAL(HAVE_GRADLE, [0])
 fi
 
 AC_SUBST(GRADLE)