Bug#1093721: gimp-help: re-enable parallel builds

[Jeremy Bícha]

Source: gimp-help
Version: 2.10.34-2
X-Debbugs-CC: andreas at fatal.se

parallel builds were disabled in 2.10.0-1 in 2020. See
https://salsa.debian.org/gnome-team/gimp-help/-/commit/cd7296a49

This significantly slows down the build and it would be nice if we
could re-enable parallel builds.

Perhaps the way forward is to switch the upstream build system from
autotools to meson, as was done with gimp, babl, and gegl.

Thank you,
Jeremy Bícha