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/
gplugin_doc_content_files = [
'embedding.md',
'genie.md',
'lua.md',
'native-plugins.md',
'perl5.md',
'python3.md',
'vala.md',
]
if get_option('doc')
gplugin_toml = configure_file(
input : 'gplugin.toml.in',
output : 'gplugin.toml',
configuration : version_conf,
install : true,
install_dir : docs_dir / 'gplugin',
)
gplugin_doc = custom_target('gplugin-doc',
input : [ gplugin_toml, gplugin_gir[0] ],
output : 'gplugin',
command : [
gidocgen,
'generate',
'--quiet',
'--fatal-warnings',
'--config=@INPUT0@',
'--output-dir=@OUTPUT@',
'--no-namespace-dir',
'--content-dir=@0@'.format(meson.current_source_dir()),
'@INPUT1@'
],
depend_files : [ gplugin_doc_content_files ],
build_by_default : true,
install : true,
install_dir : docs_dir,
)
endif