pgsql: Update docs on building for Windows to accomodate current realit

Update docs on building for Windows to accomodate current reality.

Document how to build 64 bit Windows binaries using the MinGW64 tool set.
Remove recommendation against using Mingw as a build platform.
Be more specific about when Cygwin is useful and when it's not,  in
particular note its usefulness for running psql, and
add a note about building on Cygwin in non-C locales.

Per recent discussions.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=51be78b09a83b8d533e4a9f81cf9a7f2edde6654

Modified Files
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doc/src/sgml/install-windows.sgml |   21 ++++++++++++++++-----
doc/src/sgml/installation.sgml    |   20 +++++++++++++++++++-
2 files changed, 35 insertions(+), 6 deletions(-)