pidgin/pidgin

Convert the gplugin subproject to use the auto fallback stuff of dependency.

Testing Done:
Verified the the purples gir's loaded and were able to call into gplugin under python, when built as a subproject and against a system installed version.

Reviewed at https://reviews.imfreedom.org/r/937/
FACEBOOK_SOURCES = [
'api.c',
'api.h',
'data.c',
'data.h',
'facebook.h',
'facebook.c',
'http.c',
'http.h',
'id.h',
'json.c',
'json.h',
'mqtt.c',
'mqtt.h',
'thrift.c',
'thrift.h',
'util.c',
'util.h'
]
if DYNAMIC_FACEBOOK
facebook_prpl = shared_library('facebook', FACEBOOK_SOURCES,
dependencies : [json, libpurple_dep, libsoup, glib],
install : true, install_dir : PURPLE_PLUGINDIR)
# Used to produce docs.
facebook_inc = include_directories('.')
facebook_dep = declare_dependency(
link_with : facebook_prpl,
dependencies : [json, libpurple_dep, glib])
endif