Should mata be always built as a static library?

[jurajsic]

Currently we force cmake to build mata as a static library by

add_library(libmata STATIC

If we change it to

add_library(libmata

it should by default still be built as a static library, but if BUILD_SHARED_LIBS is on, it will build a shared library.

Do we want to allow this? It might be better to allow it, if somebody wants to use mata as a shared library, so that they do not need to change the cmake file.