AC_PROG_OBJCXX succeeds without Objective C++ compiler present
AC_PROG_OBJCXX is satisfied by finding g++, but this does not mean that
cc1objplus is installed. This means that AC_PROG_OBJCXX passes, but
compilation fails with an obscure error about "execvp: not found".
This problem presumably also affects C (if it's possible to install cc1obj
without cc1), Objective C and C++.