Introduce support for the Meson build system

[pks-t]

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.

These patches have been split out of an upstream patch series as discussed.

[j6t]

Thank you. Kindly prefix commit titles with gitk: (and the name "Gitk" need then not be repeated). Other title prefixes aren't needed as the project is sufficiently small.

In this change:

 gitk-wish: gitk GIT-TCLTK-VARS
-	$(QUIET_GEN)$(RM) $@ $@+ && \
-	sed -e '1,3s|^exec .* "$$0"|exec $(subst |,'\|',$(TCLTK_PATH_SQ)) "$$0"|' <gitk >$@+ && \
-	chmod +x $@+ && \
-	mv -f $@+ $@
+	$(QUIET_GEN)$(RM) $@ $@+
+	$(QUIET_GEN)$(SHELL_PATH) ./generate-tcl.sh "$(TCLTK_PATH_SQ)" "$<" "$@"

please keep the && chain, because otherwise I see the message twice:

gitk@ps/meson-build:1019> make
    GEN gitk-wish
    GEN gitk-wish

I haven't tested the meson build. I trust it works as designed.

[pks-t]

@j6t Thanks, I've addressed both comments 👍