Java: Fix parallel make with autotools

Running 'make -j{jobs}' on a build that was configured with Java
(--with-java) would previously cause an error:

    make: *** No rule to make target `TJExample.class', needed by
        `turbojpeg.jar'.

It seems that parallel make doesn't understand that the files in
$(JAVA_CLASSES) are all generated from the same invocation of javac, so
it tries to parallelize the building of those files (which of course
doesn't work.)  This patch instead makes turbojpeg.jar depend on
classnoinst.stamp.  This effectively creates a synchronization fence,
since that file is only created when all of the class files have been
built.

Fixes #62

java/Makefile.am

@@ -44,7 +44,7 @@ JAVA_CLASSES = org/libjpegturbo/turbojpeg/TJ.class \
 
 all: all-am turbojpeg.jar
 
-turbojpeg.jar: $(JAVA_CLASSES) ${srcdir}/MANIFEST.MF
+turbojpeg.jar: classnoinst.stamp ${srcdir}/MANIFEST.MF
 	$(JAR) cfm turbojpeg.jar ${srcdir}/MANIFEST.MF $(JAVA_CLASSES)
 
 clean-local: