Fix to my fix to my fix. Use ifneq ($(XPIDLSRCS),), as used elsewhere, rather than ifdef XPIDLSRCS.

Many thanks to Chris Seawood <cls@seawood.org> both for identifying the original problem with the make rules that was causing spurious .h rebuilds, and for pointing out fixes for both of my previous attempts at getting this right.

config/rules.mk

@@ -288,7 +288,7 @@ everything:: clobber_all alldep
 #
 MAKE_DIRS =
 
-ifdef XPIDLSRCS
+ifneq ($(XPIDLSRCS),)
 MAKE_DIRS += $(XPIDL_GEN_DIR)
 endif