make liblzma reference independent of OS

Thank you for the useful package!

For linking on OS X, we need to link to `liblzma.5.dylib` (or similar), as the suffix for shared libraries is different then on Linux and other Unices. Actually, linking with "-llzma" should be sufficient for both, shouldn't it?

With the change in this pull request, I manage to install on OS X 10.9.4, linking to MacPorts' liblzma.