Support ~Darwin and support the CMake build system on Darwin, plus minor other changes

[RJVB]

This introduces a number of changes aiming to make lzvn available on platforms other than the Mac OS:

• make Mac-specific code conditional behind #ifdefs
• add a generic-Unix Makefile
• add a CMakeLists.txt file (which builds static and shared versions of the FastCompression library)
• use a .S extension on the assembly files so they can be preprocessed; use AS=cc for this in the Makefiles
• preprocessor macros are used to prefix the exported symbols with an underscore or not (should maybe be used everywhere but on Linux?)

In addition, a bug is fixed in lzvn.c that caused a crash when opening the output file failed.