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('lua')
e = executable('test-lua-loader', 'test-lua-loader.c',
c_args : [
'-DLUA_LOADER_DIR="@0@/.."'.format(meson.current_build_dir()),
'-DLUA_PLUGIN_DIR="@0@/plugins"'.format(
meson.current_source_dir()),
],
link_with : gplugin_loader_tests,
dependencies : [GLIB, GOBJECT, LUA, gplugin_dep])
test('Lua Loader', e)
# Add the test plugins path to our environment variable
devenv.append('GPLUGIN_PLUGIN_PATH', meson.current_source_dir() / 'plugins')
endif # lua