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/
SILC_SOURCES = [
'buddy.c',
'chat.c',
'ft.c',
'ft.h',
'ops.c',
'pk.c',
'silc.c',
'silcpurple.h',
'util.c',
'wb.c',
'wb.h'
]
if DYNAMIC_SILC
silc_prpl = shared_library('silcpurple', SILC_SOURCES,
dependencies : [silc, libpurple_dep, glib],
install : true, install_dir : PURPLE_PLUGINDIR)
endif