pgsql: Distinguish printf-like functions that support %m from those tha

Distinguish printf-like functions that support %m from those that don't.

The elog/ereport family of functions certainly support the %m format spec,
because they implement it "by hand". But elsewhere we have printf wrappers
that might or might not allow it depending on whether the platform's printf
does. (Most non-glibc versions don't, and notably, src/port/snprintf.c
doesn't.) Hence, rather than using the gnu_printf format archetype
interchangeably for all these functions, use it only for elog/ereport.
This will allow us to get compiler warnings for mistakes like the ones
fixed in commit a13b47a59, at least on platforms where printf doesn't
take %m and gcc is correctly configured to know it. (Unfortunately,
that won't happen on Linux, nor on macOS according to my testing.
It remains to be seen what the buildfarm's gcc-on-Windows animals will
think of this, but we may well have to rely on less-popular platforms
to warn us about unportable code of this kind.)

Discussion: https://postgr.es/m/2975.1526862605@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/3a60c8ff892a8242b907f44702bfd9f1ff877d45

Modified Files
--------------
config/c-compiler.m4 | 14 +++++++-------
configure | 6 +++---
src/include/c.h | 6 +++++-
src/include/pg_config.h.in | 4 ++--
src/include/utils/elog.h | 30 +++++++++++++++---------------
5 files changed, 32 insertions(+), 28 deletions(-)