pgsql: Add macro PGWARNING, and make PGERROR available on all platforms

Add macro PGWARNING, and make PGERROR available on all platforms.

We'd previously noted the need for coping with Windows headers
that provide some other definition of macro "ERROR" than elog.h
does. It turns out that R also wants to define ERROR, and
WARNING too. PL/R has been working around this in a hacky way
that broke when we recently changed the numeric value of ERROR.
To let them have a more future-proof solution, provide an
alternate macro PGWARNING for WARNING, and make PGERROR visible
always, not only when #ifdef WIN32.

Discussion: https://postgr.es/m/CADK3HHK6iMChd1yoOqssxBn5Z14Zar8Ztr3G-N_fuG7F8YTP3w@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d7cff12c4c035b7cf12bb8454824f48f13018730

Modified Files
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src/include/utils/elog.h | 16 +++++++++-------
1 file changed, 9 insertions(+), 7 deletions(-)