Fix earlier fixes to avoid spurious idl->.h file regeneration.

The way I did it previously breaks with parallel (-j) builds; this seems to fix it.

config/rules.mk

@@ -288,6 +288,10 @@ everything:: clobber_all alldep
 #
 MAKE_DIRS =
 
+ifdef XPIDLSRCS
+MAKE_DIRS += $(XPIDL_GEN_DIR)
+endif
+
 ifdef MDDEPDIR
 $(MDDEPDIR):
 	@if test ! -d $@; then echo Creating $@; rm -rf $@; mkdir $@; else true; fi
@@ -919,11 +923,12 @@ endif
 # Export the elements of $(XPIDLSRCS), generating .h and .xpt files and
 # moving them to the appropriate places.
 
+ifneq ($(XPIDLSRCS),)
+
 ifndef XPIDL_MODULE
 XPIDL_MODULE = $(MODULE)
 endif
 
-ifneq ($(XPIDLSRCS),)
 ifeq ($(XPIDL_MODULE),) # we need $(XPIDL_MODULE) to make $(XPIDL_MODULE).xpt
 export:: FORCE
 	@echo
@@ -941,12 +946,14 @@ export:: $(XPIDLSRCS) $(XPDIST)/idl
 	$(INSTALL) -m 444 $^
 
 # generate .h files from into $(XPIDL_GEN_DIR), then export to $(XPDIST)/include;
-# warn against overriding existing .h file.
+# warn against overriding existing .h file.  (Added to MAKE_DIRS above.)
+$(XPIDL_GEN_DIR):
+	@if test ! -d $@; then echo Creating $@; rm -rf $@; mkdir $@; else true; fi
 
 # don't depend on $(XPIDL_GEN_DIR), because the modification date changes
 # with any addition to the directory, regenerating all .h files -> everything.
+
 $(XPIDL_GEN_DIR)/%.h: %.idl $(XPIDL_COMPILE)
-	@if test ! -d $(XPIDL_GEN_DIR); then echo Creating $(XPIDL_GEN_DIR); rm -rf $(XPIDL_GEN_DIR); mkdir $(XPIDL_GEN_DIR); else true; fi
 	$(XPIDL_COMPILE) -m header -w -I $(XPDIST)/idl -I$(srcdir) -o $(XPIDL_GEN_DIR)/$* $<
 	@if test -n "$(findstring $*.h, $(EXPORTS))"; \
 	  then echo "*** WARNING: file $*.h generated from $*.idl overrides $(srcdir)/$*.h"; else true; fi
@@ -958,7 +965,6 @@ ifndef NO_GEN_XPT
 # generate intermediate .xpt files into $(XPIDL_GEN_DIR), then link
 # into $(XPIDL_MODULE).xpt and export it to $(DIST)/bin/components.
 $(XPIDL_GEN_DIR)/%.xpt: %.idl $(XPIDL_COMPILE)
-	@if test ! -d $(XPIDL_GEN_DIR); then echo Creating $(XPIDL_GEN_DIR); rm -rf $(XPIDL_GEN_DIR); mkdir $(XPIDL_GEN_DIR); else true; fi
 	$(XPIDL_COMPILE) -m typelib -w -I $(XPDIST)/idl -I$(srcdir) -o $(XPIDL_GEN_DIR)/$* $<
 
 $(XPIDL_GEN_DIR)/$(XPIDL_MODULE).xpt: $(patsubst %.idl,$(XPIDL_GEN_DIR)/%.xpt,$(XPIDLSRCS))