Fix hardcoded full path for pysam libs.

[ccoulombe]

Pysam include library directory and set libraries instead of providing full path.
Set rpath so libs are found when installed in a virtual environment or in user-space.

This allow to use pairtools in different environment and find libs at runtime instead of having them hardcoded at build time.

[agalitsyna]

HI, @ccoulombe Thanks for your PR!

1. Can you provide an example environment setting when the current version won't work?
2. How robust is setting the paths relative to $ORIGIN?

[ccoulombe]

1. The current will not work when :

• the package needs to be redistributed as a wheel
• the package is installed in a different environment then the one used to build.
  Since the full path of the libraries from the build environment are used. For instance, a wheel from PyPI or from an HPC center that redistribute this software as pre-built wheel.

2. Very robust, as this is only a complement and does not hurt to have this extra path.
   $ORIGIN always point to the origin of the library and since pysam is required to be installed along pairtools, the required libraries will always be at $PWD/../..pysam when installed in user-space or in a virtual environment since pip install packages in : lib/pythonX.Y/site-packages ( and pysam libraries live in the installation directory).
   Obviously, this does not apply when the packages are installed system-wide, in which case the linker should be able to find the pysam libraries from its config (e.g. another system-wide path).

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In the end, this only add flexibility and make the package installable in different environment by not using hard coded paths.