[PATCH 4/7] meson: use precompiled headers for unit tests

Same as in the preceding commit, our unit tests don't use precompiled headers yet. In this case though it's a tiny bit more complicated to make use of them, as we do not want to include "git-compat-util.h" for "clar.c", as that code file is a third-party implementation that is independent of the Git codebase.

But there's an easy workaround: instead of linking that file into the executable directly, we can easily adapt it to be built into a static library first. Like that we can trivially have separate build flags for that one file.

Do so and adapt the remaining sources to use precompiled headers. This results in a small but noticeable build speedup:

  Benchmark 1: meson compile (version = HEAD~)
    Time (mean ± σ):      5.343 s ±  0.019 s    [User: 75.478 s, System: 20.382 s]
    Range (min … max):    5.308 s …  5.376 s    10 runs

  Benchmark 2: meson compile (version = HEAD)
    Time (mean ± σ):      5.077 s ±  0.017 s    [User: 70.557 s, System: 19.999 s]
    Range (min … max):    5.047 s …  5.103 s    10 runs

  Summary
    meson compile (version = HEAD) ran
      1.05 ± 0.01 times faster than meson compile (version = HEAD~)

Signed-off-by: Patrick Steinhardt <ps@pks.im>
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 t/meson.build | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)