Fix etags generation

[Mike Hearn]

Maybe it's just my system, but we now have so many source files that
make etags complains about the arguments list being too long. Using
xargs fixes it.

Use xargs for etags generation to avoid argument list limit

--- orig/Makefile.in
+++ mod/Makefile.in
@@ -126,7 +126,7 @@
 # Misc rules
  
 TAGS etags:
-       etags `find $(TOPSRCDIR) -name '*.[ch]' -a -not -name '*.spec.c' -a -not -name '*.dbg.c' -print`
+       find $(TOPSRCDIR) -name '*.[ch]' -a -not -name '*.spec.c' -a -not -name '*.dbg.c' -print | xargs etags
  
 tags ctags:
        ctags --c-types=+px `find $(TOPSRCDIR) -name '*.[ch]' -a -not -name '*.spec.c' -a -not -name '*.dbg.c' -print`