WITH_CLANG_IS_CC revision 232322 
 $FreeBSD: head/tools/build/options/WITH_CLANG_IS_CC 232322 2012-02-29 22:58:51Z dim $
Set to install the Clang C/C++ compiler as

a /usr/bin/cc
,

a /usr/bin/c++
and

a /usr/bin/cpp
.