The use of the --with-cxx-main configure flag is removed along with
explicit setting of CXX.

The motivation for this came from the inability to build iminuit
(https://pypi.org/project/iminuit/) in the Docker container with the
error

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File
"/home/docker/.local/lib/python3.6/site-packages/iminuit/__init__.py",
line 27, in <module>
    from ._libiminuit import Minuit
ImportError:
/home/docker/.local/lib/python3.6/site-packages/iminuit/_libiminuit.cpython-36m-x86_64-linux-gnu.so:
undefined symbol: _ZTVN10__cxxabiv117__class_type_infoE

This seems to have come from having a mismatch between the CPython build
compiler (which was gcc in this instance) and the compiler used to build
the C++ extension modules of iminuit (which was presumably g++) due to
the hard coding with configure. Following the very helpful and detailed
information on the --with-cxx-main flag from the o...