pgsql: Document usage of COPT environment variable for adjusting config

Document usage of COPT environment variable for adjusting configure flags.

Also add to the existing rather half-baked description of PROFILE,
which does exactly the same thing, but I think people use it differently.

Discussion: /messages/by-id/16461.1487361849@sss.pgh.pa.us

Branch
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REL9_6_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/365ee96d535e91d9808d21a8a78c8664cfc44d5e

Modified Files
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doc/src/sgml/installation.sgml | 28 ++++++++++++++++++++++++++++
src/Makefile.global.in | 5 +++++
2 files changed, 33 insertions(+)

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