Allow to override the admsXml executable when invoking
autogen.sh and configure.ac
ADMSXML=/foo/bar/baz/admsXml ./autogen.sh --adms
ADMSXML=/foo/bar/baz/admsXml ./configure --enable-adms
alternatively (and more advertised)
./configure ADMSXML=/foo/bar/baz/admsXml --enable-adms
The generated Makefiles remember the given path.
A relative path doesn't make sense of course.