pgsql: 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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f1f10a1ba9e17e606a7b217ccccdd3cc4d8cb771

Modified Files
--------------
src/backend/storage/page/bufpage.c |  9 +--------
src/backend/utils/adt/lockfuncs.c  |  6 ++++++
src/backend/utils/misc/guc.c       | 39 ++++++++++++++++++++++++++++++++++++++
src/bin/pg_dump/pg_dump_sort.c     |  3 +++
src/common/relpath.c               |  3 +++
src/include/c.h                    | 18 ++++++++++++++----
src/include/storage/bufpage.h      | 10 ++++++++++
7 files changed, 76 insertions(+), 12 deletions(-)