widl: Add IDL installation directory to include path.

tools/widl/Makefile.in

@@ -21,6 +21,7 @@ C_SRCS = \
 LEX_SRCS   = parser.l
 BISON_SRCS = parser.y
 
+EXTRADEFS   = -DDEFAULT_INCLUDE_DIR=\"$(includedir)/windows/\"
 INSTALLDIRS = $(DESTDIR)$(bindir) $(DESTDIR)$(mandir)/man$(prog_manext)
 
 all: $(PROGRAMS)

tools/widl/widl.c

@@ -643,6 +643,10 @@ int main(int argc,char *argv[])
     }
   }
 
+#ifdef DEFAULT_INCLUDE_DIR
+  wpp_add_include_path(DEFAULT_INCLUDE_DIR);
+#endif
+
   if(do_everything) {
     set_everything(TRUE);
   }