--with-cxx-include-arch and --with-cxx-include-64bit-dir
It was blocking the automatic detection of the path of clang.
In particular in the context of the usage of -target.
However, it does not completely fix the detection of the i386 C++ path.
See the next item.
(Closes: #729933)
* Bring back the path to libstdc++ under i386. Still not fixed upstream
(Closes: #730857)
