Instructions about building with libclang in README.md are outdated

If you use GCC 5 or higher on Linux, please do not use our LLVM package, but get
the package for your distribution. Our LLVM package is compiled with GCC 4, so
you get linking errors, because GCC 5 is using a C++ 11 conforming string
implementation, which is not used by GCC 4. To sum it up, do not mix GCC 5 and
GCC 4 binaries. On Ubuntu, you can download the package from
http://apt.llvm.org/ with:

 ...

There is a workaround to set _GLIBCXX_USE_CXX11_ABI to 1 or 0, but we recommend
to download the package from http://apt.llvm.org/.