Check for fallocate at runtime

On linux, if during configure, we find posix_fallocate, but not fallocate,
we try the syscall for fallocate to see if it works.  Going back as far as
RHEL 5.5, the syscall is in fact there and working, but not in libc.
Merge pull request #1386 from erh/fallocate_syscall_check

Check for fallocate at runtime