Shared library support

[Klaim]

If I did it right:

• CI should build and test both static and shared versions of libpowerloader (might be broken right now, will need to check the cI);
• Use POWERLOADER_API in libpowerloader headers to mark functions (and functions in classes) that needs to be exported symbols;
• Makes shared library build the default if BUILD_SHARED_LIBS is not specified;

[JohanMabille]

Looks good to me, the only missing part is binary versionning, but that might be added later. See here and here fo an example of how we set it in xeus.

[Klaim]

Ah yes, that would be the focus of a separate PR indeed.