The .in.in file was added to the repo in 8bdfa6a4e56f. Then Gary added the .in file to POTFILES.in in 1d98555... but I think it should actually be the .in.in file, right?
# Makefile.mingw
#
# Description: Makefile to generate mo files
#
PIDGIN_TREE_TOP:=..
include $(PIDGIN_TREE_TOP)/libpurple/win32/global.mak