Coccinelle for PostgreSQL development [3/N]: meson support

The third patch adds support to Meson by adding a coccicheck target. Since
ninja (which is used in the chapter "Building and Installation with
Meson"[1]) does not support variables in the same way as make does, it is
currently necessary to use:

MODE=patch ninja coccicheck

An alternative is to have three explicit targets "coccicheck-report",
"coccicheck-context", and "coccicheck-patch", which might be easier to use.
It does not work if you want to pass other flags, however, so you would
still have to use something like this if you want to pass in extra flags:

SPFLAGS=--debug ninja coccicheck-context

[1]: https://www.postgresql.org/docs/current/install-meson.html
--
Best wishes,
Mats Kindahl, Timescale