This tells Meson that we intend libzstd_dep to be used by a parent project
if the parent looks for a dependency named "libzstd". Without this, the
mapping from "libzstd" to our variable libzstd_dep must be encoded in the
Meson wrap file or in the parent's meson.build.
Meson always returns -pthread in dependency('threads') on non-MSVC
compilers. On Windows we use Windows threading primitives, so we don't
need this. Avoid adding -pthread to libzstd's link flags, either as a
Meson subproject or via pkg-config Libs.private, so the application
doesn't inadvertently depend on winpthreads.
Add a Meson MinGW cross-compile CI test that checks for this. It turns
out that pzstd fails to build in that environment, so have the test
skip building contrib for now.
Benjamin Gilbert