oslib-posix: check for posix_memalign in configure script

Check for the presence of posix_memalign() in the configure script,
not using "defined(_POSIX_C_SOURCE) && !defined(__sun__)".  This
lets qemu use posix_memalign() on NetBSD versions that have it,
instead of falling back to valloc() which is wasteful when the
required alignment is smaller than a page.

Signed-off-by: Andreas Gustafsson <gson@gson.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Kamil Rytarowski <n54@gmx.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>