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Installing Ruby on MacOS 11.6 produces compilation errors in the downloaded source; how do I fix?

(If MacOS is not sufficiently Linux-y for this community, please let me know and I'll ask this elsewhere.)

A project I want to work on requires Ruby.  I was directed to [RVM](https://rvm.io/) or [rbenv](https://github.com/rbenv/rbenv#installation).  I tried `rbenv` first because it has Mac instructions, ran into failures, uninstalled it and installed RVM, and still ran into problems.  I am not sure how to debug.  Details:

I am on MacOS 11.6 (Big Sur).  I am using the terminal (bash command line).  I'll jump to the problem with RVM because the developer I'm working with uses it.

I installed RVM using the following command (from their documentation): 

    \curl -sSL https://get.rvm.io | bash -s stable

That did not produce any error messages.  This installation modified my .bash_profile, so I fired up a new shell.  In it I used `rvm list known` and confirmed that the list includes `[ruby-]2.6[.6]`, the version my project needs.  I then typed:

    rvm install "ruby-2.6.6"

Which produced a compilation error (substituting USER for my actual user name in paths):

    Checking requirements for osx.
    Certificates bundle '/usr/local/etc/openssl@1.1/cert.pem' is already up to date.
    Requirements installation successful.
    Installing Ruby from source to: /Users/USER/.rvm/rubies/ruby-2.6.6, this may take a while depending on your cpu(s)...
    ruby-2.6.6 - #downloading ruby-2.6.6, this may take a while depending on your connection...
      % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                     Dload  Upload   Total   Spent    Left  Speed
    100 13.4M  100 13.4M    0     0  7961k      0  0:00:01  0:00:01 --:--:-- 7957k
    ruby-2.6.6 - #extracting ruby-2.6.6 to /Users/USER/.rvm/src/ruby-2.6.6.....
    ruby-2.6.6 - #configuring......................................................|
    ruby-2.6.6 - #post-configuration.
    ruby-2.6.6 - #compiling...................................................
    Error running '__rvm_make -j8',
    please read /Users/USER/.rvm/log/1637706281_ruby-2.6.6/make.log
    There has been an error while running make. Halting the installation.

I consulted the log file given at the end, where I found this error:

    compiling date_core.c
    compiling digest.c
    readline.c:1905:37: error: use of undeclared identifier 'username_completion_function'; did you mean 'rl_username_completion_function'?
                                        rl_username_completion_function);
                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                                        rl_username_completion_function
    readline.c:79:42: note: expanded from macro 'rl_username_completion_function'
    # define rl_username_completion_function username_completion_function
                                         ^
    /usr/local/opt/readline/include/readline/readline.h:485:14: note: 'rl_username_completion_function' declared here
    extern char *rl_username_completion_function PARAMS((const char *, int));
                 ^
    1 error generated.
    make[2]: *** [readline.o] Error 1
    make[1]: *** [ext/readline/all] Error 2

My attempt to install using `rbenv` ended with a very similar error, so switching from RVM to `rbenv` won't solve my problem.

How can I remedy this problem?  I'm wondering if these Ruby managers require something in my environment that I'm missing, despite the output saying that dependencies were checked.  Maybe it's assuming a different compiler than whatever I have?

This error isn't coming from *my* code; it's coming from the Ruby source code that RVM downloaded and tried to build.  I guess I could try debugging that code, if I knew anything about it, but that seems like the wrong approach.  I'm trying to get the "official" Ruby 2.6.6 to run, not my locally-hacked Ruby 2.6.6.

I don't know Ruby or its environment; I'm hoping to learn as I work on this project.  Because of that lack of experience, though, I don't know where to look next or how to debug this problem.  How do I successfully install this version of Ruby?