Fix distribution of dx-stamp.m4.

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.