Use "stamp" files to sequence javac and javah invocations.

In my build log, I noticed that javah was invoked multiple times, as yocto's bitbake invokes make with the -j flag.

This change introduces the "stamp" file idiom, where a stamp file is used to represent the make targets with multiple files.  While I only truely needed it for javah, I added a stamp file for javac too. This allows the javah invocation to be dispatched at the same time as jar.