meson: simplify version_tag handling

meson: simplify version_tag handling

Let's also use vcs_tag() when we're doing a non-git build. In those scenarios,
the build would normally be done just once in a given copy, so doing an extra
call does not matter. We can save a few lines of meson config.

The special path was added in 064b8e2c99ceb348c515353cc5c7d7bd05c49fcb, with
the justifaction that vcs_tag() is slow and -Dversion-tag=foo can be used to
fix the version tag and speed up partial rebuilds. I think the justification
for this is weak: having an accurate version tag is particularly useful when
developing the code. Shaving of a fraction of a second at the cost of having to
manually update the version seems iffy.

Secondly, with vcs_tag() we can be pretty sure that meson will build the
version file first and that it'll be available to all build steps. Because we
didn't use version tag, we had to manually specify the dependency on version.h
in various places. It seems nicer to use vcs_tag() and not have to deal with
this problem at all.

Finally, the savings in time seem much smaller than back when
064b8e2c99ceb348c515353cc5c7d7bd05c49fcb was made. It reported a change
from 94 ms to 521 ms. But now the difference seems to be about 50 ms:

Before this patch:
$ time ninja -C build
ninja: Entering directory `build'
ninja: no work to do.
ninja -C build  0.04s user 0.02s system 97% cpu 0.057 total
ninja -C build  0.03s user 0.01s system 97% cpu 0.049 total
ninja -C build  0.03s user 0.02s system 96% cpu 0.051 total
ninja -C build  0.03s user 0.01s system 96% cpu 0.049 total
ninja -C build  0.03s user 0.01s system 97% cpu 0.046 total

With the two patches in this PR:
systemd-stable [drop-versiondep] time ninja -C build
ninja: Entering directory `build'
[1/669] Generating version.h with a custom command
ninja -C build  0.08s user 0.03s system 98% cpu 0.106 total
ninja -C build  0.08s user 0.03s system 98% cpu 0.104 total
ninja -C build  0.09s user 0.02s system 98% cpu 0.116 total
ninja -C build  0.08s user 0.02s system 97% cpu 0.108 total

Overall, I think the tiny time savings are not worth the complexity.