xbps-src/build-dependencies.sh: check for instead of

common/xbps-src/shutils/build_dependencies.sh

@@ -240,7 +240,7 @@ install_pkg_deps() {
                 continue
             else
                 echo "   [host] ${i}: not found."
-                if [ -z "$CROSS_BUILD" ]; then
+                if [ -z "$cross" ]; then
                     if [ "${_realpkg}" = "$targetpkg" ]; then
                         msg_error "${pkg}: [host] build loop detected: ${_realpkg} <-> ${targetpkg} [depends on itself]\n"
                     elif [ "${_realpkg}" = "$pkg" ]; then