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?
EXTRA_DIST=\
COPYRIGHT\
ChangeLog.API\
ChangeLog.win32\
HACKING\
Makefile.mingw\
PLUGIN_HOWTO\
README.hg\
README.mingw\
config.h.mingw\
intltool-extract.in\
intltool-merge.in\
intltool-update.in\
m4macros/introspection.m4\
package_revision.h\
po/Makefile.mingw\
tap-driver.sh\
tap-test\
valgrind-suppressions
noinst_HEADERS=config.hpackage_revision.h
dist-pre-hook:package_revision.h
if !ENABLE_GTK_DOC
$(AM_V_at)echo"error: gtk-doc must be installed and enabled in order to make dist"
$(AM_V_at)echo"re-run ./configure with --enable-gtk-doc switch and try again"