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/
if ENABLE_DOC
subdir('reference')
endif
if ENABLE_GTK
pidgin_man = configure_file(
input : 'pidgin3.1.in',
output : 'pidgin3.1',
configuration : man_conf,
install : true,
install_dir : get_option('mandir') / 'man1')
endif
if enable_consoleui
finch_man = configure_file(
input : 'finch3.1.in',
output : 'finch3.1',
configuration : man_conf,
install : true,
install_dir : get_option('mandir') / 'man1')
endif