configure rebuild rule not passing on -I to aclocal

[Warren Young]

My bootstrap script conditionally adds a -I flag to the aclocal command
on Mac platforms when it sees that we've installed the Homebrew version
of gettext (for AM_ICONV) since for reasons I forget, it works better
for us than the the platform one. The logic looks like this:

if [ -d /usr/local/Cellar/gettext ]
then
ald=`echo /usr/local/Cellar/gettext/*/share/aclocal`
export ACLOCAL_FLAGS="-I $ald"
fi

aclocal -I config $ACLOCAL_FLAGS && ....

This works fine *until* one of the automatic rebuild rules gets
triggered. You see it try to run aclocal like this:

.... && aclocal -I config -I config

One of the '-I config's comes from my AC_CONFIG_MACRO_DIR([config])
directive in configure.ac. I don't know if it's just repeating that or
getting another '-I config' from somewhere else.

More to the point, I don't see how to get it to pass on the -I flag my
bootstrap script gave to the initial run of aclocal. Shouldn't Autoconf
pick up on the exported ACLOCAL_FLAGS variable and append its value here?

A bit of Googling suggests I might be able to fix this with the
AC_CONFIG_MACRO_DIRS (plural) form, but it appears to be undocumented.

I've been able to "fix" this by adding "AM_MAINTAINER_MODE" in
configure.ac to disable the automatic rebuild rules, but that feels like
a dirty hack. I like what the default rebuild rules try to do; I don't
want to disable them. I just need to figure out how to make them pass
on flags I've already told the Autotools about once before. :)