pgsql: Add infrastructure for compile-time assertions about variable ty

Add infrastructure for compile-time assertions about variable types.

Currently, the macros only work with fairly recent gcc versions, but there
is room to expand them to other compilers that have comparable features.

Heavily revised and autoconfiscated version of a patch by Andres Freund.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/ea473fb2dee7dfe61f8fbdfac9d9da87d84e564e

Modified Files
--------------
config/c-compiler.m4 | 40 ++++++++++++++
configure | 119 +++++++++++++++++++++++++++++++++++++++++
configure.in | 2 +
src/include/c.h | 41 ++++++++++++++
src/include/pg_config.h.in | 6 ++
src/include/pg_config.h.win32 | 6 ++
6 files changed, 214 insertions(+), 0 deletions(-)