Add option for optionally building shared lib
Add option for optionally building shared lib

CMakeLists.txt:
Add `RTOSC_BUILD_SHARED_LIBS` option to optionally build librtosc and
librtosc_cpp as shared library by setting the global variable
`BUILD_SHARED_LIBS` to ON and removing the specific `STATIC` option from
the `add_library()` calls.
This way static libraries are built by default and shared libraries can
be built by using `-D RTOSC_BUILD_SHARED_LIBS=ON`.

Fixes #40

fundamental/rtosc,fundamental/rtosc,fundamental/rtosc