pgsql: Assume that we have signed integral types and flexible array mem

Assume that we have signed integral types and flexible array members.

These compiler features are required by C99, so remove the configure
probes for them.

This is part of a series of commits to get rid of no-longer-relevant
configure checks and dead src/port/ code. I'm committing them separately
to make it easier to back out individual changes if they prove less
portable than I expect.

Discussion: https://postgr.es/m/15379.1582221614@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f4d59369d2ddf0ad7850112752ec42fd115825d4

Modified Files
--------------
config/c-compiler.m4 | 15 ----------
configure | 72 ----------------------------------------------
configure.in | 2 --
src/include/c.h | 10 +++++++
src/include/pg_config.h.in | 12 --------
src/tools/msvc/Solution.pm | 2 --
6 files changed, 10 insertions(+), 103 deletions(-)