Use relative SO lookup for mathsat extension
Use relative SO lookup for mathsat extension

On linux, e.g. this translates to using `--rpath=$ORIGIN` for
`_mathsat` extension binary, so `libmathsat.so` is found in `bindings_dir`,
without the need to set `LD_LIBRARY_PATH`.

On Windows, DLL loading happens relative to the binary anyway.

pysmt/pysmt,pysmt/pysmt