Summary:
Completes the transition to disabled legacy support by default across all build systems. This follows up on the previous Makefile and CMake changes to ensure consistent default behavior regardless of the build system used.
Updated build configurations: Meson, tests/Makefile, Visual Studio 2008/2010 projects, and BUCK.
Test Plan:
Verified changes compile correctly via `make lib-release`. Build system configurations have been updated consistently across all platforms.
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.
