pgsql: Allow MSVC to build with Tcl 8.6.

Allow MSVC to build with Tcl 8.6.

Commit eaba54c20c5 added support for Tcl 8.6 for configure-supported
platforms after verifying that pltcl works without further changes, but
the MSVC tooling wasn't updated accordingly. Update MSVC to match,
restructuring the code to avoid duplicating the logic for every Tcl
version supported.

Backpatch to all live branches, like eaba54c20c5. In 9.4 and previous,
change the patch to use backslashes rather than forward, as in the rest
of the file.

Reported by Paresh More, who also tested the patch I provided.
Discussion: https://postgr.es/m/CAAgiCNGVw3ssBtSi3ZNstrz5k00ax=UV+_ZEHUeW_LMSGL2sew@mail.gmail.com

Branch
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REL9_2_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/992e581bff3b0984e23d3fae5e9bed62a5d226fd

Modified Files
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src/tools/msvc/Mkvcbuild.pm | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)