upgrade gcc + compiling ROOT - c++

Question

I am working on ROOT v6.06.06 on ubuntu 14.04. Everything went OK.
Lately i did apt-get upgrade, and this affected the c++ compiler.
Since then, when i try to run my macros on ROOT, I have this error:
fatal error: file '/usr/include/c++/4.9/cmath' modified since it was first processed
I tried then to re-compile ROOT, but it's not working, I have this error:
[ 36%] Building CXX object interpreter/llvm/src/tools/clang/lib/Driver/CMakeFiles/clangDriver.dir/ToolChains.cpp.o
.../root/interpreter/llvm/src/tools/clang/lib/Driver/ToolChains.cpp: In static member function ‘static std::string clang::driver::toolchains::Hexagon_TC::GetGnuDir(const string&, const llvm::opt::ArgList&)’:
.../root/root6/root/interpreter/llvm/src/tools/clang/lib/Driver/ToolChains.cpp:2110:42: error: ‘LLVM_PREFIX’ was not declared in this scope
std::string PrefixRelDir = std::string(LLVM_PREFIX) + "/../gnu";
^
make[2]: *** [interpreter/llvm/src/tools/clang/lib/Driver/CMakeFiles/clangDriver.dir/ToolChains.cpp.o] Error 1
make[1]: *** [interpreter/llvm/src/tools/clang/lib/Driver/CMakeFiles/clangDriver.dir/all] Error 2
make: *** [all] Error 2
So I need either solving the c++ compiler so i can use the last compiled version of ROOT that I have,
Or succeed to solve the problem while compiling ROOT.
Can you help me please? I am blocked. Thanks a lot.