meson: do a full usage-based compile check for sysinfo

meson: do a full usage-based compile check for sysinfo

On Solaris, sys/sysinfo.h is a completely different file and doesn't
resemble the linux file at all. There is also a sysinfo() function, but
it takes a totally different call signature, which asks for:

- the field you wish to receive
- a `char *buf` to copy the data to

and is very useful IFF you want to know, say, the hardware provider. Or,
get *specific* fields from uname(2).

https://docs.oracle.com/cd/E86824_01/html/E54765/sysinfo-2.html

It is surely possible to do this manually via `sysconf(3)` without the
nice API. I can't find anything more direct. Either way, I'm not very
attached to Solaris, so someone who cares can add it. Either way, it's
wrong to assume that sysinfo.h contains what we are looking for.

Check that sysinfo.h defines the struct we actually utilize in
builtins/gc.c, which will correctly fail on systems that don't have it.

Signed-off-by: Eli Schwartz <eschwartz@gentoo.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>