] 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: 8a6b1665d9 ("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>

m4/set_cflags_ldflags.m4

@@ -15,7 +15,7 @@ for ldflag in $APPEND_LIB
 do
     APPEND_LDFLAGS="$APPEND_LDFLAGS -L$ldflag"
 done
-if [ ! -z $EXTRA_PREFIX ]; then
+if test ! -z "$EXTRA_PREFIX" ; then
     CPPFLAGS="$CPPFLAGS -I$EXTRA_PREFIX/include"
     LDFLAGS="$LDFLAGS -L$EXTRA_PREFIX/lib"
 fi