xen: fix gcc6 build again

srcpkgs/xen/template

@@ -45,8 +45,8 @@ nostrip_files="
 pycompile_module="xen grub"
 
 pre_configure() {
-	local i _gccver=$(gcc --version | awk '/^gcc \(GCC\)/ { print $3 }')
-	[ "${_gccver%%.*}" -lt 6 ] && return 0
+	local i
+	[ ${XBPS_GCC_VERSION_MAJOR} -lt 6 ] && return 0
 
 	sed -i tools/configure -e's;-Werror;-Wno-error;g'
 	# Copy patches for _ipxe.tar.gz contents