meson: check for getpagesize before using it

meson: check for getpagesize before using it

It is deprecated and removed in SUS v3 / POSIX 2001, so various systems
may not include it. Solaris, in particular, carefully refrains from
defining it except inside of a maze of `#ifdef` to make sure you have
kept your nose clean and only used it in code that *targets* SUS v2 or
earlier.

config.mak.uname defines this automatically, though only for QNX.

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