meson: Respect --default-library option

By using shared_library() we force build of only a shared library, change it to library() to build static, shared or both depending on what user asked in build options.

Also fix generation of pkg-config file in the case gtksourceview is built as subproject. It is rarely needed to put anything in 'requires', meson knows if public libraries have a pkg-config name it can use. Also meson knows private dependencies, no need to specify them.