Set the proper path for the shared library so that anything linking
against it will be able to find the library.

Go, and likely other language bindings will, need access to create
new combinators. This functionality is currently located
src/parsers/parser_internal.h. Install this header to the system with
dependent headers until a better solution can be found.

Create a pkg-config file and install it to $prefix/lib/pkgconfig

Don't pass the entire environment to SCons. Only pass the PATH

Don't clober the Environment (and thus the ENV['PATH'] by redefining it
and don't link against lr by default. Only add it if uname is not Darwin