Log Message:
emulators/qemu: check for posix_memalign in configure script

qemu incorrectly checks for the presence of posix_memalign() by
testing whether the preprocessor symbol _POSIX_C_SOURCE is defined.
This causes qemu to use valloc() instead of posix_memalign() on NetBSD,
even on versions where posix_memalign() is supported, wasting
memory when the required alignment is smaller than a page.  Fix this
by checking for posix_memalign() support in the configure script.