libtool: Unset CXX. It is not needed to build libtool, but if set configure...

libtool: Unset CXX. It is not needed to build libtool, but if set configure preferes CXX over CC to build the global_symbol_pipe test program. So if there is no c++ compiler or the c++ installation is broken we end up with a broken libtool script. Fixes #5022

SVN-Revision: 16212