Commit b589696 added lines to some shell within AC_ARG_WITH macros, but
inadvertently failed to move the final closing ). e.g.:

    OPT_RUSTLS=no
    AC_ARG_WITH(rustls,dnl
    AS_HELP_STRING([--with-rustls=PATH],[where to look for rustls, PATH points to the installation root]),
      OPT_RUSTLS=$withval
      if test X"$withval" != Xno; then
        test -z "TLSCHOICE" || TLSCHOICE="${TLSCHOICE:+$TLSCHOICE, }rustls")
        experimental="$experimental rustls"
      fi

This generates a broken looking configure script:

    # Check whether --with-rustls was given.
    if test "${with_rustls+set}" = set; then :
      withval=$with_rustls; OPT_RUSTLS=$withval
      if test X"$withval" != Xno; then
        test -z "TLSCHOICE" || TLSCHOICE="${TLSCHOICE:+$TLSCHOICE
    else
      }rustls"
    fi

        experimental="$experimental rustls"
      fi

But amazingly, it executes OK, due to balancing " quotes and if/fi, as
you can see if you reformat the white space:

    # Check whether --with-rustls was given.
    if test "${with_rustls+set}" = set; then :
      withval=$with_rustls; OPT_RUSTLS=$withval
      if test X"$withval" != Xno; then
        test -z "TLSCHOICE" || TLSCHOICE="${TLSCHOICE:+$TLSCHOICE else }rustls"
      fi

      experimental="$experimental rustls"
    fi

We move the closing ) back to the end of the section of script:

    OPT_RUSTLS=no
    AC_ARG_WITH(rustls,dnl
    AS_HELP_STRING([--with-rustls=PATH],[where to look for rustls, PATH points to the installation root]),
      OPT_RUSTLS=$withval
      if test X"$withval" != Xno; then
        test -z "TLSCHOICE" || TLSCHOICE="${TLSCHOICE:+$TLSCHOICE, }rustls"
        experimental="$experimental rustls"
      fi
    )

However, with this change alone the generated configure script is still broken:

    # Check whether --with-rustls was given.
    if test "${with_rustls+set}" = set; then :
      withval=$with_rustls; OPT_RUSTLS=$withval
      if test X"$withval" != Xno; then
        test -z "TLSCHOICE" || TLSCHOICE="${TLSCHOICE:+$TLSCHOICE
    else
      }rustls"
        experimental="$experimental rustls"
      fi

    fi

This is because there is an unquoted comma in the script,
unintentionally separating AC_ARG_WITH arguments. We need to quote the
script section using braces:

    OPT_RUSTLS=no
    AC_ARG_WITH(rustls,dnl
    AS_HELP_STRING([--with-rustls=PATH],[where to look for rustls, PATH points to the installation root]),[
      OPT_RUSTLS=$withval
      if test X"$withval" != Xno; then
        test -z "TLSCHOICE" || TLSCHOICE="${TLSCHOICE:+$TLSCHOICE, }rustls"
        experimental="$experimental rustls"
      fi
    ])

The generated script now has sane shell syntax:

    # Check whether --with-rustls was given.
    if test "${with_rustls+set}" = set; then :
      withval=$with_rustls;
      OPT_RUSTLS=$withval
      if test X"$withval" != Xno; then
        test -z "TLSCHOICE" || TLSCHOICE="${TLSCHOICE:+$TLSCHOICE, }rustls"
        experimental="$experimental rustls"
      fi

    fi

However, the 'test -z "TLSCHOICE" ||' part is a no-op, the same as
'false ||'. Perhaps it was cut-and-pasted from somewhere else, and a
dollar before TLSCHOICE was accidentaly deleted. However, with that
change, TLSCHOICE would never be set.

The code on the rhs of the || makes sense - it appends the new value,
separating it from any previous value with ", ". So, we can simplify by
removing the no-op test. This gives us:

    OPT_RUSTLS=no
    AC_ARG_WITH(rustls,dnl
    AS_HELP_STRING([--with-rustls=PATH],[where to look for rustls, PATH points to the installation root]),[
      OPT_RUSTLS=$withval
      if test X"$withval" != Xno; then
        TLSCHOICE="${TLSCHOICE:+$TLSCHOICE, }rustls"
        experimental="$experimental rustls"
      fi
    ])

Generating:

    # Check whether --with-rustls was given.
    if test "${with_rustls+set}" = set; then :
      withval=$with_rustls;
      OPT_RUSTLS=$withval
      if test X"$withval" != Xno; then
        TLSCHOICE="${TLSCHOICE:+$TLSCHOICE, }rustls"
        experimental="$experimental rustls"
      fi

    fi

As far as I can tell, the quoting problem and no-op test have been there
for as long as this code has existed.

So, if these problems have been around for a while, how did I find them?
Only because I did a configure including these options:

    $ ./configure --with-openssl --without-rustls
      SSL:              enabled (OpenSSL)

and got this diagnostic at the end:
      WARNING:  rustls enabled but marked EXPERIMENTAL. Use with caution!

I've tested these changes by running various combinations of options,
and checking that the diagnostic at the end prints correctly. e.g.:

    $ ./configure --without-ssl
      SSL:              no      (--with-{openssl,gnutls,nss,mbedtls,wolfssl,schannel,secure-transport,amissl,bearssl,rustls} )

    $ ./configure --with-openssl
      SSL:              enabled (OpenSSL)

    $ ./configure --with-openssl --with-gnutls
      SSL:              enabled (OpenSSL, GnuTLS)