Use $(SED) to make sure GNU sed is used

FreeBSD's default sed is not compatible with this expression.

Reviewed-by: Daniel Stone <daniels@collabora.com>

Makefile.am

@@ -1619,7 +1619,7 @@ $(if $(findstring unstable,$1),unstable,stable)
 endef
 
 define protoname
-$(shell echo $1 | sed 's/\([a-z\-]\+\)-[a-z]\+-v[0-9]\+/\1/')
+$(shell echo $1 | $(SED) 's/\([a-z\-]\+\)-[a-z]\+-v[0-9]\+/\1/')
 endef
 
 protocol/%-protocol.c : $(WAYLAND_PROTOCOLS_DATADIR)/$$(call protostability,$$*)/$$(call protoname,$$*)/$$*.xml