gplugin/gplugin

Don't explicitly use shared_library on gplugin, gplugin-gtk3, and gplugin-gtk4

By just using the library function meson can choose to static link these into
another target.

Testing Done:
Did intial testing in the pidgin tree by hacking `gplugin/meson.build` and used `ldd` to verify that gplugin was staticly linked and that plugins still worked.

Reviewed at https://reviews.imfreedom.org/r/1346/
if get_option('vapi')
if not get_option('introspection')
error('Vala generation requires GObject Introspection.')
endif
add_languages('vala', native: false)
gplugin_vapi = gnome.generate_vapi('gplugin',
sources : gplugin_gir[0],
install : true,
)
if get_option('gtk3')
gplugin_gtk_vapi = gnome.generate_vapi('gplugin-gtk3',
sources : gplugin_gtk3_gir[0],
packages : [ 'gtk+-3.0' ],
install : true,
gir_dirs : meson.current_build_dir() / '..' / 'gplugin',
)
endif
subdir('tests')
endif # vala