Previously there was a check that made it possible for the users to
switch between building a static or shared library when using Pistache
as a Meson subproject. Now Meson checks for the desired library type
before defining any build targets, and if somebody wants to only use the
static library it doesn't have to also build the shared one.

Also, code is now way cleaner, since the two calls to static_library and
shared_library are now a single library() call