I agree with Pablo Galindo Salgado: https://bugs.python.org/issue35912#msg334942

The "quick and dirty" solution is to change MAINCC to CC, for _testembed.c AND python.c (g++ fails with both).

After that, _testembed.c and python.c should be changed so they can be compiled with a c++ compiler, and a system test should be added.

Anyway, I found the original patch:
https://bugs.python.org/file6816/cxx-main.patch

In the original patch, the README contained detailed information. I think these informations could be restored, maybe in ./configure --help

Anyway, I have a question. In README, it's stated:

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.

All x86 platforms? Also x86-64? And what does it means "Linux with ELF"? It means that Linux has shared libraries or that Python is compiled with --enable-shared? And what it means gcc 3 and 4? It means gcc 3+?