libjclang and jextract prototype

[Henry Jen]

Hi,

Because JNR is not in panama yet, I implemented a small libclang binding 
via JNI enough to support jextract experiment.

As we would like to continue the experiment/exploration, it's probably a 
good idea to have the development tracked in the panama repo, thus I 
prepared a webrev with build changes, you can see the webrev at:

http://cr.openjdk.java.net/~henryjen/panama/jextract

README have a short brief on how one can get libclang and direct 
configure to find the header files.

I took the liberty of using current package name and location, which can 
be adjusted with feedbacks.

If I can get a quick review and approval, I'll commit them to panama repo.

Cheers,
Henry