A recent gettext release (0.25 via Homebrew) installs the M4 macros in a
different location (<prefix>/share/gettext/m4 instead of
<prefix>/share/aclocal). According to the commit messages to avoid "bad
interactions between autoreconf and autopoint". Since we only depend
on gettext for that macro and this move makes it complicated, we can also
just integrate the macro from gnulib directly (which gettext 0.18+ relies
on anyway).
Use a Debian-based Docker container to run the unit tests for charon-tkm,
once without and once with TKM running. The container can also be used
locally to run the tests (see comments in the Dockerfile).
