aclocal.m4 cannot be generated by aclocal
Since rev 52202, ruby added a aclocal.m4 file to its source. Normally, it is expected that aclocal.m4 to be generated by command aclocal that reads m4 directory. In order to avoid using tainted autotools-generated files, some build env removes all autotools-generated files, touch sources and regenerate the autotools files. As aclocal.m4 is hand-written and required by Makefile, compilation fails. This is the case of OpenWRT but it might happen for other envs.
It would be interesting to have aclocal.m4 macros be incorporated into another file or generated by aclocal command.