We regret the delay in responding to this report.

Your suggested changes do not make sense to me.  There should be no behavioral difference between

m4_bmatch([$1], [^/.*], [...])

and

m4_bmatch([$1], [^/], [...])

because m4_bmatch does not require the pattern to match the entire string.  And anything that could match \$\{?[A-Za-z] should have been sent to the other branch of the AS_LITERAL_WORD_IF.

Could you please provide some examples of uses of AC_CONFIG_AUX_DIR that do not work correctly for you with the code in 2.71?  As many as you have.

AR_CONFIG_AUX_DIR macro in general.m4 does seem to have insufficient regex, and does therefore not correspond to its description which is

_# AC_CONFIG_AUX_DIR(DIR)
# ----------------------
# Find auxiliary scripts (e.g. install-sh, config.sub, config.guess)
# in DIR.  If DIR is a literal shell word and not an absolute path,
# it is interpreted relative to $srcdir; otherwise it is assumed to be
# usable as-is._

I experimented with a custom macro:

AC_DEFUN([MY_CONFIG_AUX_DIR],
  [m4_append_uniq([_AC_AUX_DIR_CANDIDATES],
    AS_LITERAL_WORD_IF([$1],
      [m4_bmatch([$1],
                 [^/.*],       [$1],      # added '.*' suffix
                 [^[a-z]:/], [$1],        # is that to match shell vars?
                            [${srcdir}/$1])],
      [$1]),
    [${PATH_SEPARATOR}])])