* CMake: version sofiles and respect libdir

Version the libraries like libnsync.so.1.20.0 and set the symlinks
properly. The version is taken from the VERSION file in the root, it
should be updated with new releases. The minimum is kept CMake version
is kept at 2.8 but >=3.0 is strongly recommended. Since the minimum is
<3.0, policy CMP0048 must be set to NEW.

64bit distros use /usr/lib64 not /usr/lib so this needs to be
configurable as well.

The source properties for C++ are set outside of enabling the tests
otherwise building the C++ library with tests disabled fails.

Signed-off-by: Jason Zaman <jason@perfinion.com>

* CMake: add separate libnsync_cpp library target so C/C++ do not conflict

The libraries should be named differently otherwise they cannot be
installed together on the system. libnsync.so is the C library.
libnsync_cpp.so is the C++ library. The header files for both are
identical.

Both are built from the same source files so the C++ source files must
be copied so that the source file property LANGUAGE can be set. The copy
is done during build so the configure step does not need to be re-done
after any code changes. Per-OS settings are now all done together and
then two functions set the C / C++ properties on the build targets.

The C library is the recommended one and the C++ library takes
significantly longer to build and test so it is possible to only build
and install only the C library.

$ mkdir build; cd build
$ cmake ..
$ make nsync
$ make test_c
$ make install

Will build and install only the C library. make all will still build
both libraries like normal. There are also test_c / test_cpp targets
to run only C/C++ tests.

Signed-off-by: Jason Zaman <jason@perfinion.com>