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 old Python 2.7 SVN
trunk README (https://svn.python.org/projects/python/trunk/README) this
became more clear that not only was --with-cxx-main not wanted but that
setting CXX was unnecessary especially as c++ and g++ are the same on
Ubuntu:

> --with-cxx-main=<compiler>: If you plan to use C++ extension modules,
>        then -- on some platforms -- you need to compile python's main()
>        function with the C++ compiler. With this option, make will use
>        <compiler> to compile main() *and* to link the python executable.
>        It is likely that the resulting executable depends on the C++
>        runtime library of <compiler>. (The default is --without-cxx-main.)
>
>        There are platforms that do not require you to build Python
>        with a C++ compiler in order to use C++ extension modules.
>        E.g., x86 Linux with ELF shared binaries and GCC 3.x, 4.x is such
>        a platform. We recommend that you configure Python
>        --without-cxx-main on those platforms because a mismatch
>        between the C++ compiler version used to build Python and to
>        build a C++ extension module is likely to cause a crash at
>        runtime.
>
>        The Python installation also stores the variable CXX that
>        determines, e.g., the C++ compiler distutils calls by default
>        to build C++ extensions. If you set CXX on the configure command
>        line to any string of non-zero length, then configure won't
>        change CXX. If you do not preset CXX but pass
>        --with-cxx-main=<compiler>, then configure sets CXX=<compiler>.
>        In all other cases, configure looks for a C++ compiler by
>        some common names (c++, g++, gcc, CC, cxx, cc++, cl) and sets
>        CXX to the first compiler it finds. If it does not find any
>        C++ compiler, then it sets CXX="".
>
>        Similarly, if you want to change the command used to link the
>        python executable, then set LINKCC on the configure command line.

Also of some note is CPython Issue 23644 (https://bugs.python.org/issue23644)
as it enforces the idea of not trying to force CXX versions.

A final note, if the CC and CXX are left unset the values that were used
can be determined from sysconfig using the following:

python3 -c "import sysconfig; print(sysconfig.get_config_vars('CC', 'CXX', 'LDCXXSHARED'))"

which in this container yields

['gcc -pthread', 'g++ -pthread', 'g++ -pthread -shared']