Fix RPATH in p4c-barefoot

[vgurevich]

I see that install_batch.sh sets LD_LIBRARY_PATH to $SDE_INSTALL/lib.

This is OK, but was never required with the original SDE and I think it is only needed because we build the compiler from scratch.

It might be better to fix RPATH in the compiler executable and do not require that extra environment variable.

The procedure is:

sudo apt-get install -y patchelf
patchelf --set-rpath $SDE_INSTALL/lib p4c-barefoot

[fruffy]

We can easily set this in CMake: https://stackoverflow.com/a/43551534

But I need to check the right place for this.

[ChrisDodd]

Just a suggestion: it is better to use a $$ORIGIN based rpath rather than an absolute rpath -- that way things can continue to work in docker containers or chroot setting which change the absolute path.