Backport part of Pidgin revision 35800b680594b4a9a13085c006dbde401d9ab541
(including the fix in 1acb1cc0f7e5947393c4af3ef1a2cc7266aa1317):
"I also made it so any custom AUTO*_FLAGS specified will override the default
arguments instead of just appending to them." --nosnilmot