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/
[library]
version = "@GPLUGIN_VERSION@"
browse_url = "https://keep.imfreedom.org/gplugin/gplugin/"
repository_url = "https://keep.imfreedom.org/gplugin/gplugin/"
website_url = "https://keep.imfreedom.org/gplugin/gplugin/"
authors = "GPlugin Developers"
logo_url = ""
license = "LGPL-2.1-or-later"
description = "GPlugin Gtk Library"
dependencies = [ "GLib-2.0", "GObject-2.0", "GPlugin-1.0", "Gtk-4.0" ]
devhelp = true
search_index = true
[dependencies."GLib-2.0"]
name = "GLib"
description = "General-purpose, portable utility library."
docs_url = "https://docs.gtk.org/glib/"
[dependencies."GObject-2.0"]
name = "GObject"
description = "The base type system library"
docs_url = "https://docs.gtk.org/gobject/"
[dependencies."GPlugin-1.0"]
name = "GPlugin"
description = "Plugin library"
docs_url = "https://docs.imfreedom.org/gplugin/"
[dependencies."Gtk-4.0"]
name = "Gtk4"
description = "GTK Widget Toolkit"
docs_url = "https://docs.gtk.org/gtk4/"
[theme]
name = "basic"
show_index_summary = true
show_class_hierarchy = true
[source-location]
base_url = "https://keep.imfreedom.org/gplugin/gplugin/file/default/"
[extra]
# The same order will be used when generating the index
content_files = [
]
content_images = [
]
urlmap_file = "urlmap.js"