meson: catch up to a few configure changes

meson: catch up to a few configure changes

I (Andres) missed a few recent changes to configure when merging
e6927270cd1 "meson: Add initial version of meson based build system". Mirror
the changes in
- ec3c9cc202f "Add definition pg_attribute_aligned() for MSVC"
- b086a47a270 "Bump minimum version of Bison to 2.3"
- 8b878bffa8d "Bump minimum version of Flex to 2.5.35"

As MSVC does not implement 128 bit integers, the oversight of not using
pg_attribute_aligned() should not have current practical consequences. But of
course the code from c.h should still be correctly mirrored.

I (Andres) also hadn't implemented the minimum perl version check. Added that
now.

Reported-by: Junwang Zhao <zhjwpku@gmail.com>
Author: Junwang Zhao <zhjwpku@gmail.com>
Author: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAEG8a3K9c87EwAwmdOgmS0Li1J6P_7r-Uc0-zN6cJtrMr7VvPg@mail.gmail.com