make install getting slower

Hi,

best of three of
time make -s install INSTALL=/bin/true

9.3: 1.53s
9.4: 1.55s
9.5: 1.90s
9.6: 2.10s
10: 2.45s
11: 2.81s
master: 2.99s

Obviously a part of that can be explained by the tree getting bigger,
but that can't be all of it.

It also gets a bit worse if LLVM is enabled (due to the bitcode files
getting installed), but that's "just" 0.3s for me. We should optimize
that for fewer install invocations / using cp, but that seems seperate.

I think this also has gotten more pronounced with newer OS versions, as
the overhead of the spectre/meltdown mitigations have increased the cost
of syscalls and forks.

Obviously this doesn't terribly matter for actual installations, but I
do find it fairly annoying when running regression tests...

Greetings,

Andres Freund