Update makefile to add $JAVA_HOME/include/linux to JAVA_INCLUDES on Linu...

During the build an #include of jni_md.h fails.  This patch fixes this by adding an additional path to $JAVA_INCLUDES.

This fix is from http://stackoverflow.com/questions/13466777/jni-h-no-such-file-or-directory, and seems the least intrusive possible.  It should be safe as this part of the makefile only applies to Linux.

Alternative solutions (ie change your environment) are described at https://bugs.openjdk.java.net/browse/JDK-7173867.