I noticed this error with the autoheader as shown below:

 * Applying xulrunner-1.9.2-gtk+-2.21.patch ...                         [ ok ]
 * Running eautoreconf in '/var/tmp/portage/www-
client/firefox-3.6.12/work/mozilla-1.9.2' ...
 * Running autoconf ...                                                 [ ok ]
 * Running autoheader ...                                               [ !! ]
 * Running elibtoolize in: 
mozilla-1.9.2/ipc/chromium/src/third_party/libevent/
 *   Applying install-sh-1.5.patch ...
 *   Applying portage-1.5.10.patch ...
 *   Applying sed-1.5.6.patch ...
 *   Applying as-needed-1.5.26.patch ...
 * Running elibtoolize in: mozilla-1.9.2/js/ctypes/libffi/
 *   Applying install-sh-1.5.4.patch ...
 *   Applying portage-1.5.10.patch ...
 *   Applying sed-1.5.6.patch ...
 *   Applying as-needed-1.5.26.patch ...
 *   Applying uclibc-ltconf-1.3.0.patch ...
 * Running elibtoolize in: mozilla-1.9.2/modules/freetype2/builds/unix/
 *   Applying portage-2.2.patch ...
 *   Applying sed-1.5.6.patch ...
 *   Applying as-needed-2.2.6.patch ...
 * Running elibtoolize in: mozilla-1.9.2/toolkit/crashreporter/google-
breakpad/autotools/
 *   Applying portage-2.2.patch ...
 *   Applying sed-1.5.6.patch ...
 *   Applying as-needed-2.2.6.patch ...
 * Running eautoreconf in '/var/tmp/portage/www-
client/firefox-3.6.12/work/mozilla-1.9.2/js/src' ...
* Running autoconf ...                                                  [ ok ]
 * Running autoheader ...                                               [ !! ]
>>> Source prepared.

This is an amd64 box.  I can't recall seeing the same on a x86 machine earlier 
in the week - but may have just missed it.

Is it important?
-- 
Regards,
Mick