libtool version 2.2.6 only checks Makefile.am for ACLOCAL_AMFLAGS.

[Robin Holt]

I have a package which has all of its Makefiles for the actual package
named GNUmakefile.am.  libtoolize prints a failure message because it is
only checking Makefile.am for the ACLOCAL_AMFLAGS=-I m4.  I changed
/usr/bin/libtoolize lines 1243 and 1245 to use GNUmakefile.am instead of
Makefile.am and the error went away.

Thanks,
Robin Holt