gplugin/gplugin

Add gir targets to the dependencies sources attribute and other meson cleanups.

Adding the gir targs to the dependency object of the library allows us to be
able to use the dependency in a gnome.generate_gir for a library that consumes
our api regardless of if it's a system install or a subproject.

Testing Done:
Modified the gplugin subproject in pidgin and used these changes to verify that it fixed the current build issues over there.

Reviewed at https://reviews.imfreedom.org/r/939/
option(
'doc',
type : 'boolean', value : true, yield : true,
description : 'build documentation with gtk-doc'
)
option(
'introspection',
type : 'boolean', value : true, yield : true,
description : 'Whether or not to build a GObject Introspection type library'
)
option(
'gtk3',
type : 'boolean', value : true,
description : 'Whether or not to build the gtk3 library'
)
option(
'help2man',
type : 'boolean', value : true,
description : 'Whether or not to build man pages from --help output'
)
option(
'install-gplugin-gtk-viewer',
type : 'boolean', value : true,
description : 'Whether or not to install the gtk viewer application'
)
option(
'install-gplugin-query',
type : 'boolean', value : true,
description : 'Whether or not to install the query application'
)
option(
'lua',
type : 'boolean', value : true,
description : 'Whether or not to build the Lua plugin loader'
)
option(
'nls',
type : 'boolean', value : true,
description : 'Install translation files'
)
option(
'perl5',
type : 'boolean', value : true,
description : 'Whether or not to build the Perl plugin loader'
)
option(
'python3',
type : 'boolean', value : true,
description : 'Whether or not to build the Python 3.x plugin loader'
)
option(
'rs',
type : 'boolean', value : false,
description : 'Whether or not to create the rust bindings.'
)
option(
'tcc',
type : 'boolean', value : false,
description : 'Whether or not to build the TCC plugin loader'
)
option(
'vapi',
type : 'boolean', value : true,
description : 'Whether or not to build vapi files for gplugin'
)