configure: Don't run Xen compile checks in subshells

The Xen compile checks are currently run inside subshells. This
is unnecessary and has the effect that if do_cc() exits with
an error message then this only causes the subshell to exit,
not the whole of configure, which is confusing. Remove the
subshells, changing:
  if ( cat ; compile_prog ) ; then ...
to
  if cat && compile_prog ; then ...

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
