Upstream Git has introduced support for the Meson build system.
Introduce support for Meson into gitk, as well, so that Git can easily
build its vendored copy of Gitk via a `subproject()` directive. The
instructions can be set up as follows:
$ meson setup build
$ meson compile -C build
$ meson install -C build
Specific options, like for example where Gitk shall be installed to, can
be specified at setup time via `-D`. Available options can be discovered
by running `meson configure` either in the source or build directory.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
b4c06f7c4d