Backport the new 10.9+ installer variant from 3.7.  This variant features
more modern options; like 64-bit only (Apple is deprecating 32-bit support
in future macOS releases); a built-in version of Tcl/Tk 8.6.8; built with
clang rather than gcc-4.2.  For 3.6.5, the 10.9+ variant will be offered
as an additional alternative to the traditional 10.6+ variant in earlier
3.6.x releases.  Binary extension modules (including wheels) built for
earlier versions of 3.6.x with the 10.6 variant should continue to work
with either 3.6.5 variant without recompilation.

In addition, both installer variants have updated 3rd-party libraries:
OpenSSL 1.0.2m -> 1.0.2n
XZ 5.2.2 -> 5.2.3
SQLite 3.21.0 -> 3.22.0

Also the 10.6 variant now sets CC=gcc instead of CC=gcc-4.2 and does not
search for the outdated 10.6 SDK.  The variant is built with the same
compiler as before.  As before, for extension module builds, the CC can
be overridden with the CC env variable and an SDK can be specified
with the SDKROOT env variable (see man xcrun).  These minor changes
should be transparent to nearly all users.