meson: precompile "git-compat-util.h"

meson: precompile "git-compat-util.h"

Every compilation unit in Git is expected to include "git-compat-util.h"
first, either directly or indirectly via "builtin.h". This header papers
over differences between platforms so that we can expect the typical
POSIX functions to exist. Furthermore, it provides functionality that we
end up using everywhere.

This header is thus quite heavy as a consequence. Preprocessing it as a
standalone unit via `clang -E git-compat-util.h` yields over 23,000
lines of code overall. Naturally, it takes quite some time to compile
all of this.

Luckily, this is exactly the kind of use case that precompiled headers
aim to solve: instead of recompiling it every single time, we compile it
once and then link the result into the executable. If include guards are
set up properly it means that the file won't need to be reprocessed.

Set up such a precompiled header for "git-compat-util.h" and wire it up
via Meson. This causes Meson to implicitly include the precompiled
header in all compilation units. With GCC and Clang for example this is
done via the "-include" statement [1].

This leads to a significant speedup when performing full builds:

  Benchmark 1: ninja (rev = HEAD~)
  Time (mean ± σ):     14.467 s ±  0.126 s    [User: 248.133 s, System: 31.298 s]
  Range (min … max):   14.195 s … 14.633 s    10 runs

  Benchmark 2: ninja (rev = HEAD)
    Time (mean ± σ):     10.307 s ±  0.111 s    [User: 173.290 s, System: 23.998 s]
    Range (min … max):   10.030 s … 10.433 s    10 runs

  Summary
    ninja (rev = HEAD) ran
      1.40 ± 0.02 times faster than ninja (rev = HEAD~)

[1]: https://gcc.gnu.org/onlinedocs/gcc/Precompiled-Headers.html

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>