configure: Centralize check for posix_fallocate

configure: Centralize check for posix_fallocate

This checks for posix_fallocate unless we are sitting on an ancient glibc.
With this we don't need HAVE_BROKEN_POSIX_FALLOCATE anymore,
HAVE_POSIX_FALLOCATE will only be defined if we have a valid [g]libc.

./configure tested on Debian, FreeBSD (which does have posix_fallocate) and
OpenBSD (which does not have posix_fallocate). Also tested with changing the
not have an old-enough glibc around. All did the right thing.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>