libmypaint: fix build (#424557)
`dev` output unchanged (verified by `diffoscope`).
`out` output still contains all the translation files,
with no obvious encoding issues i spotted. Slight
deviations in the metadata, but should be fine.
Fixing the build with modern gettext requires replacing the obsolete glib gettext.
Fixing the build with modern autoconf requires making IT_PROG_INTLTOOL
unconditional, hence the indent changes in the patch.