Makefile: hide stderr of curl-config test

You will get $ make distclean 2>&1 | grep curl /bin/sh: curl-config: not found /bin/sh: curl-config: not found /bin/sh: curl-config: not found /bin/sh: curl-config: not found /bin/sh: curl-config: not found $ if you don't have a curl development package installed. The intent is not to alarm the user, but just to test if there is a new enough curl installed. However, if you look at search engine suggested completions, the above "error" messages are confusing people into thinking curl is a hard requirement. Redirect this error output to /dev/null as it is not necessary to be shown to the end users. Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>