The automatic distribution of dx-stamp.m4 depends on m4_include
appearing in the m4 traces. Normally aclocal does that but it
seems to not be entirely reliable. The reason we do not simply
use m4_include is because this produces warnings about multiple
inclusion.
Fortunately, this is pretty easy to work around. Just use m4_pushdef
to redefine m4_include to what we need, so the traces show the file
needs to be distributed.