WITH_CLANG_IS_CC revision 235342 
 $FreeBSD: head/tools/build/options/WITH_CLANG_IS_CC 235342 2012-05-12 16:12:36Z gjb $
Set to install the Clang C/C++ compiler as

a /usr/bin/cc ,

a /usr/bin/c++
and

a /usr/bin/cpp .