[Scons-dev] compiler lookups, particularly windows

[Mats Wichmann]

Yesterday I tested a small change which improves finding an alternate
build suite on windows (visual studio being considered primary, build
tools being "other").  In running the tests, I unintentionally left a
debug statement in the file and in the log saw this:

XXX FOUND VC: C:\Program Files (x86)\Microsoft Visual
Studio\2017\BuildTools\VC

a total of 1519 times.

some of this process is kind of expensive... once the lookup that
generated that message has worked, one of the "vcvars" scripts is run to
find the actual needed executable and any other details - there's
already a comment in the code this is an expensive operation ("not
cheap"), and there's some effort to speed it up.

Is it worth trying to figure out any way to do better at this?  It makes
test runs on Windows slow, and in particular it means the Windows CI
build, which is four runs back to back for different Python versions
take a really long time - to the point where often when you go see if
your build passed, Appveyor is waiting to even start a build because
some other build hasn't finished yet.