bpo-45433: Do not link libpython against libcrypt

[floppym]

Save/restore LIBS when calling AC_SEARCH_LIBS(..., crypt). This avoid
linking libpython with libcrypt.

https://bugs.python.org/issue45433

[vstinner]

Would you mind to add a NEWS entry? See https://devguide.python.org/

IMO it's worth it to mention the change in the "Build Changes" of What's New in Python 3.11: Doc/whatsnew/3.11.rst.

[floppym]

Added the NEWS entry.

[vstinner]

Can you please also mention the change in the "Build Changes" of What's New in Python 3.11: Doc/whatsnew/3.11.rst?

[floppym]

Sorry, I thought adding the NEWS entry would somehow auto-update the whatsnew document. I added an entry to the latter.

[vstinner]

How can I check https://bugs.python.org/issue45433 manually?

On Fedora, /usr/lib64/libpython3.10.so.1.0 is not linked to libcrypt.

[floppym]

I think most distros build with LDFLAGS="-Wl,--as-needed". That causes unused libraries to get automatically dropped by the linker.

There are a couple of alternate things to check:

python3.10-config --libs
pkg-config --libs --static python-3.10

[vstinner]

I tested manually this PR and I confirm that it works as expected.

I build Python with ./configure --with-pydebug --enable-shared && make.

Without this change, libpython is linked to libcrypt by mistake:

$ ldd libpython3.11d.so
	linux-vdso.so.1 (0x00007fff4f370000)
	libcrypt.so.2 => /lib64/libcrypt.so.2 (0x00007fb4eb5e5000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007fb4eb5c4000)
	libdl.so.2 => /lib64/libdl.so.2 (0x00007fb4eb5bd000)
	...

With this change, libcrypt is gone as expected:

$ ldd libpython3.11d.so
	linux-vdso.so.1 (0x00007fff74162000)
	libpthread.so.0 => /lib64/libpthread.so.0 (0x00007f063ee18000)
	libdl.so.2 => /lib64/libdl.so.2 (0x00007f063ee11000)
	...

[vstinner]

Nicely spotted, thanks for the fix!

I prefer to not backport to avoid any risk of regression. In my experience, the build system is fragile.