Add declaration-level assertions for compile-time checks

Add declaration-level assertions for compile-time checks

Those new assertions can be used at file scope, outside of any function
for compilation checks.  This commit provides implementations for C and
C++, and fallback implementations.

Author: Peter Smith
Reviewed-by: Andres Freund, Kyotaro Horiguchi, Dagfinn Ilmari Mannsåker,
Michael Paquier
Discussion: https://postgr.es/m/201DD0641B056142AC8C6645EC1B5F62014B8E8030@SYD1217