llvm dev - May 2014 - [LLVMdev] LLVM_PREFIX not declare

Hello,
I am installing cilkplus/llvm but after 89% of installation,I encountered
an error which says "/llvm/tools/clang/lib/Driver/ToolChains.cpp:1510:42:
error: ‘LLVM_PREFIX’ was not declared in this scope" which is this line
"std::string PrefixRelDir = std::string(LLVM_PREFIX) +
"/../gnu";". I look
at the file but couldn't find out the problem and Google too but problem
yet not fixed. I don't know what it means. I will be very helpful, I could
get some help from here!!!!!
Thank you.
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And I am getting these two errors too.

make[2]: ***
[tools/clang/lib/Driver/CMakeFiles/clangDriver.dir/ToolChains.cpp.o] Error 1
make[1]: *** [tools/clang/lib/Driver/CMakeFiles/clangDriver.dir/all] Error 2

​please help me to fix these errors.
Thank you
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