[autotools] Use correct m4 file name (umfpack.m4)

Previously a non existent file name (umfpack) was used in
Makefile.am. This broke the build with the message:

make[2]: *** Keine Regel vorhanden, um das Target »umfpack«,
  benötigt von »all-am«, zu erstellen.  Schluss.
make[2]: Leaving directory `/home/mblatt/src/dune/current/dune-common/build-auto/m4'

This patch fixes this by using the correct name.