build: don't use -s flag for `which`

`which -s` is a BSDism that doesn't exist on other versions of
`which`. That means that even on Mac OS X, `make osx` breaks if you have
another utils package installed (e.g. debianutils installed thru
fink). Redirect output to /dev/null instead.

Makefile

@@ -135,7 +135,7 b' i18n/hg.pot: $(PYFILES) $(DOCFILES) i18n'
 # Packaging targets
 
 osx:
-	@which -s bdist_mpkg || \
+	@which bdist_mpkg >/dev/null || \
 	   (echo "Missing bdist_mpkg (easy_install bdist_mpkg)"; false)
 	bdist_mpkg setup.py
 	mkdir -p packages/osx