meson: Mostly assume clang-cl is MSVC

We need to enable building the dirent and gnulib sources for clang-cl,
as we are still using the Microsoft-style headers and lib's and CRT.
We need to also do this for the following, for similar reasoning:

-Symbol export (via __declspec(dllexport))
-Dependency discovery without pkg-config files
-long long and ssize_t detection

We do, however, enable the autoptr tests for clang-cl builds.  Note that
at this point real MSVC builds are still better supported than clang-cl
builds, and it will likely remain so for at least the near future,
alhtough real MSVC builds of the GTK stack are consumable and are usable
by clang-cl.
12 jobs for clang-cl-support in 20 minutes and 48 seconds (queued for 2 seconds)