pgsql: Provide overflow safe integer math inline functions.

Provide overflow safe integer math inline functions.

It's not easy to get signed integer overflow checks correct and
fast. Therefore abstract the necessary infrastructure into a common
header providing addition, subtraction and multiplication for 16, 32,
64 bit signed integers.

The new macros aren't yet used, but a followup commit will convert
several open coded overflow checks.

Author: Andres Freund, with some code stolen from Greg Stark
Reviewed-By: Robert Haas
Discussion: https://postgr.es/m/20171024103954.ztmatprlglz3rwke@alap3.anarazel.de

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4d6ad31257adaf8a51e1c4377d96afa656d9165f

Modified Files
--------------
config/c-compiler.m4          |  22 ++++
configure                     |  33 ++++++
configure.in                  |   4 +
src/include/common/int.h      | 239 ++++++++++++++++++++++++++++++++++++++++++
src/include/pg_config.h.in    |   3 +
src/include/pg_config.h.win32 |   3 +
6 files changed, 304 insertions(+)