pyo3-build-config: For abi3, link to libpython3.so on Unix

[ravenexp]

Quoting PEP 384:

If Python is compiled as a shared library, it is installed as both
libpython3.so, and libpython3.y.so; applications conforming to this PEP
should then link to the former (extension modules can continue to link
with no libpython shared object, but rather rely on runtime linking).

Source: https://peps.python.org/pep-0384/#linkage

[ravenexp]

Previous discussion: #2324 (comment)

[ravenexp]

Ok, I'm not sure if -lpython3 even works with modern gcc toolchains...

When compiling a C program that only calls Py_Initialize():

$ gcc test.c -lpython3
/usr/bin/ld: /tmp/ccQwlIiy.o: undefined reference to symbol 'Py_Initialize'
/usr/bin/ld: /usr/lib/libpython3.10.so.1.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

Adding a workaround makes it kinda work:

$ gcc test.c -Wl,--copy-dt-needed-entries -lpython3
$ ./a.out
$ ldd a.out
        linux-vdso.so.1 (0x00007ffc38b86000)
        libpython3.so => /usr/lib/libpython3.so (0x00007f19c8d47000)
        libc.so.6 => /usr/lib/libc.so.6 (0x00007f19c8b3d000)
        libpython3.10.so.1.0 => /usr/lib/libpython3.10.so.1.0 (0x00007f19c8782000)
        /lib64/ld-linux-x86-64.so.2 => /usr/lib64/ld-linux-x86-64.so.2 (0x00007f19c8d6f000)
        libm.so.6 => /usr/lib/libm.so.6 (0x00007f19c869a000)

but, it only results in the executable linking to both libpython3.so and libpython3.10.so.

I'm going to conclude that this part of PEP 384 is dead on arrival, because I couldn't find anyone complaining about it not working as intended except for https://bugs.python.org/issue11347.