Today, I tried to build Python 3.9 from source, which failed. Building it without optimizations enabled fails due to an Segmentation fault (output_without.txt) and building with optimizations fails with a profile-opt error (output.txt).

$ lsb_release -a
No LSB modules are available.
Distributor ID:	Debian
Description:	Debian GNU/Linux bullseye/sid
Release:	9.11
Codename:	stretch

What ./configure options did you use?  Did you do a make clean or equivalent between the two build attempts?  If you used --enable-shared, try without it.

My commands were as follows:

$ ./configure
$ make &> output_without.txt
$ ./configure --enable-optimizations
$ make &> output.txt

Doing a make clean before each make resolved the issue. Thanks Ned!
I searched through the README.md on GitHub and found a note about make clean. However, the Developer's Guide at python.org doesn't mention it in the Quick References. Should it be added?

Glad you got it working.  In general, it is probably a good idea to do a "make clean" before or after re-running ./configure in a particular build directory.   Ideally, the Makefile dependencies should detect and do the right thing automatically so that a manual "make clean" would not be necessary but, with as complicated a build system as we have, that goal might be unrealistic (PRs welcome though!). In any case adding something to the devguide steps would also be a good idea. Again, an issue and/or a PR against the devguide would be welcome (https://github.com/python/devguide).