Reported by Bjarni Ingi Gislason <bjarniig@rhi.hi.is> in
<https://lists.gnu.org/archive/html/bug-gnulib/2021-12/msg00112.html>.
* gnulib-tool (func_import): Mention an AC_CONFIG_MACRO_DIRS invocation
as an alternative to augmenting ACLOCAL_AMFLAGS.
(--import, --add-import, --remove-import, --update): To find the m4
directories, look also for AC_CONFIG_MACRO_DIR and AC_CONFIG_MACRO_DIRS
invocations in configure.ac.