build: use PCH to reduce compile time

[memchr]

Using precompiled headers can consistently reduce initial build time by approximately 26%, and incremental build time by 30% to 40% in relative terms. However, this comes at the cost of increased disk usage during compilation.

For example, on a single zen2 core, compilation time, not including the linking process:

PCH	initial build	incremental build	total object files generated
no	238.22s	36.839s	244.8 MiB
yes	186.16s	22.202s	505.6 MiB

During meson setup PCH can be disabled with b_pch=false

meson setup build -Db_pch=false

During cmake setup PCH can be disabled with CMAKE_DISABLE_PRECOMPILE_HEADERS=ON

cmake -S . -B ./build -DCMAKE_DISABLE_PRECOMPILE_HEADERS=ON

Caveats

Using precompiled headers makes compilation of source files in src/ uncacheable by ccache

However, this is unlikely to have a significant impact, as the use of global variables and frequent header changes often invalidate the previous cache, leading to a full rebuild.

[vaxerski]

ok from cmake side

[vaxerski]

fuffie got drafted and he should be back next week I believe

[fufexan]

LGTM. Reduced my build time from 42s to 33s. Considering Nix builds are always "initial", this is a great reduction.