meson: Don't define HAVE_PTHREAD only on linux

meson: Don't define HAVE_PTHREAD only on linux

I'm not sure of the reason for this. I don't see anything like this in
configure.ac

In include/c11/threads.h the cases are:

1) building for Windows -> threads_win32.h
2) HAVE_PTHREAD -> threads_posix.h
3) Not supported on this platform

So not defining HAVE_PTHREAD for anything not Windows just means we can't
build at all.

When we are building for Windows, I'm not sure if dependency('threads')
would ever find anything, or defining HAVE_PTHREAD has any effect, but avoid
defining it there, just in case.

Signed-off-by: Jon Turney <jon.turney@dronecode.org.uk>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>