meson.build: Don't misdetect posix_memalign() on Windows

meson.build: Don't misdetect posix_memalign() on Windows

Currently we incorrectly think that posix_memalign() exists on
Windows.  This is because of a combination of:

 * the msys2/mingw toolchain/libc claim to have a
   __builtin_posix_memalign when there isn't a builtin of that name
 * meson will assume that if you have a __builtin_foo that
   counts for has_function('foo')

Specifying a specific include file via prefix: causes meson to not
treat builtins as sufficient and actually look for the function
itself; see this meson pull request which added that as the official
way to get the right answer:
  https://github.com/mesonbuild/meson/pull/1150

Currently this misdectection doesn't cause problems because we only
use CONFIG_POSIX_MEMALIGN in oslib-posix.c; however that will change
in a following commit.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220226180723.1706285-6-peter.maydell@linaro.org
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>