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. :)