$ ls /var/tmp/portage/sys-devel/llvm-3.2-r1/work/llvm-3.2_build/bin/ clang-tblgen This sucks a fair bit.
set(LLVM_TOOLS_BINARY_DIR /var/tmp/portage/sys-devel/llvm-3.2-r1/work/llvm-3.2_build/bin) in /usr/share/llvm/cmake/LLVMConfig.cmake would be responsible for that.