Make the Makefile.linux build system flexible enough for distribution packaging

[musicinmybrain]

These changes allow me to do the following without additional patching, other than https://github.com/Danny02/OpenCTM/pull/18:

• Link against system copies of all libraries bundled in tools/
• Respect the distribution-wide compiler and linker flags
• Version the shared library, https://github.com/Danny02/OpenCTM/issues/16

(I’m not saying what’s proposed here is beautiful, but it seems to be workable.)

If development resumes at some point, and particularly if the transition to CMake is finished, it would be nice if the above needs could be supported from the beginning.