tools: use system installed libaio by default.

tools: use system installed libaio by default.

I could have sworn I did this years ago.

IIRC the need for our own copy was due to the use of io_set_eventfd which is
not present in version 0.3.106. However it is in 0.3.107 the first version of
which was uploaded to Debian in June 2008 (I can't find a better reference for
the release date).

The necessary version is available in Debian Lenny onwards and is in at least
RHEL 6, Fedora 13 and OpenSuSE 11.3. The necessary version appears to not be
available in RHEL 5 or SLES 11 which is why I haven't simply nuked the in tree
version.

This is based on tools-system-libaio.diff from the Debian packaging although I
have made it optional (but default on).

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>