pgsql: Ensure SIZE_MAX can be used throughout our code.

Ensure SIZE_MAX can be used throughout our code.

Pre-C99 platforms may lack <stdint.h> and thereby SIZE_MAX. We have
a couple of places using the hack "(size_t) -1" as a fallback, but
it wasn't universally available; which means the code added in commit
2e70d6b5e fails to compile everywhere. Move that hack to c.h so that
we can rely on having SIZE_MAX everywhere.

Per discussion, it'd be a good idea to make the macro's value safe
for use in #if-tests, but that will take a bit more work. This is
just a quick expedient to get the buildfarm green again.

Back-patch to all supported branches, like the previous commit.

Discussion: https://postgr.es/m/15883.1504278595@sss.pgh.pa.us

Branch
------
REL9_5_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/bf387028554f45bf0af926525d6db12e7ee4281e

Modified Files
--------------
src/include/c.h | 5 +++++
src/include/utils/memutils.h | 2 +-
src/timezone/private.h | 4 ----
3 files changed, 6 insertions(+), 5 deletions(-)