COMP: External_curl - Properly set curl_DIR on multi-config system

The output directory is expected to be:

<CURL_DIR>/lib/{Release, Debug, ..}

and not

<CURL_DIR>/{Release, Debug, ..}/lib

git-svn-id: http://svn.slicer.org/Slicer4/trunk@22731 3bd1e089-480b-0410-8dfb-8563597acbee