Shared library builds worked, but failed tests because they were made
with the wrong CRT linker settings. Strings allocated within the
library could not be freed outside the library because the dynamic CRT
was not used.
This sets necessary gyp variables to link with a dynamic CRT on
Windows, thereby fixing tests running in shared library mode that
otherwise hung in a GitHub Actions environment.
Related to #318 (shared_library builds on Windows)
Issue #336 (progress toward GitHub Actions workflow to replace Travis
and Appveyor, where we need to build and test shared_library on all
platforms)
b/190743862 (internal; tracking replacement of Travis)
Change-Id: Iffefd27c2aa4ec479ce1d10b099483e417d2231f
098f58a143