] m4: use test instead of []

] m4: use test instead of []

It is reported that [] was removed by autoconf, which caused the
following error:

  ./configure: line 4681: -z: command not found

Switch to test. That's what is used throughout our configure scripts.
Also put the variable expansion in quotes.

Signed-off-by: Wei Liu <wl@xen.org>
Reported-by: Bertrand Marquis <Bertrand.Marquis@arm.com>
Fixes: 8a6b1665d987 ("configure: also add EXTRA_PREFIX to {CPP/LD}FLAGS")
Signed-off-by: Wei Liu <wl@xen.org>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>
Release-acked-by: Paul Durrant <paul@xen.org>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>