pgsql: Make numeric form of PG version number readily available in Make

Make numeric form of PG version number readily available in Makefiles.

Expose PG_VERSION_NUM (e.g., "90600") as a Make variable; but for
consistency with the other Make variables holding similar info,
call the variable just VERSION_NUM not PG_VERSION_NUM.

There was some discussion of making this value available as a pg_config
value as well. However, that would entail substantially more work than
this two-line patch. Given that there was not exactly universal consensus
that we need this at all, let's just do a minimal amount of work for now.

Back-patch of commit a5d489ccb7e613c7ca3be6141092b8c1d2c13fa7, so that this
variable is actually useful for its intended purpose sometime before 2020.

Michael Paquier, reviewed by Pavel Stehule

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/29ff43adfbfdd072b1d1aa3846ec01ff3108b666

Modified Files
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configure | 2 ++
configure.in | 1 +
src/Makefile.global.in | 1 +
3 files changed, 4 insertions(+)