libxl: explicitly check for libmath in autoconf

libxl: explicitly check for libmath in autoconf

As well as explicitly add -lm to libxl's Makefile.

This is because next patch uses floating point arithmetic, and
it is better to state it clearly that we need libmath (just in
case we find a libc  that wants that to be explicitly enforced).

Notice that autoconf should be rerun after applying this change.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Acked-by: Roger Pau Monne <roger.pau@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- s/libmath/libm/ in error message ]
Committed-by: Ian Campbell <ian.campbell@citrix.com>