I won’t get into the whys or the wherefores, but I needed to build Python 3.9.1 for Solaris 10, using gcc-5.5. I attempted the build, using the guide found here, but ran into an issue with setup.py, preventing the building of the socket module.
To get past this, I patched setup.py, with the patch I posted here.
I then used the following configure line, to set up the build:
$ ./configure --prefix=/opt/python3 --with-openssl=/opt/csw/ --enable-optimizations LDFLAGS='-L/opt/local/lib -I/opt/csw/include/ncurses -I/opt/csw/include -L/opt/csw/lib -R/opt/local/lib' PKG_CONFIG_PATH=/opt/csw/lib/amd64/pkgconfig/ CPPFLAGS='-L/opt/local/lib -I/opt/csw/include -I/opt/csw/include/ncurses -L/opt/csw/lib -R/opt/local/lib'
After that, everything worked fine.
