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('doc')
gplugin_gtk4_toml = configure_file(
input : 'gplugin-gtk4.toml.in',
output : 'gplugin-gtk4.toml',
configuration : version_conf,
install : true,
install_dir : docs_dir / 'gplugin-gtk4',
)
gplugin_gtk4_doc = custom_target('gplugin-gtk4-doc',
input : [ gplugin_gtk4_toml, gplugin_gtk4_gir[0] ],
output : 'gplugin-gtk4',
command : [
gidocgen,
'generate',
'--quiet',
'--fatal-warnings',
'--add-include-path=@0@'.format(meson.project_build_root() / 'gplugin'),
'--config=@INPUT0@',
'--output-dir=@OUTPUT@',
'--no-namespace-dir',
'--content-dir=@0@'.format(meson.current_source_dir()),
'@INPUT1@'
],
depends : [ gplugin_gir[0] ],
build_by_default : true,
install : true,
install_dir : docs_dir,
)
endif