build: Avoid lots of duplications in Makefile.

build: Avoid lots of duplications in Makefile.

We define a number of make variables, each of which becomes a cpp
define as well as an m4 macro via suitable -D var=val options for
cpp and m4. With the current approach we repeat the name of each
variables twice. Example:

	URL := http://people.tuebingen.mpg.de/maan/$(PACKAGE)/
	M4 += -D "URL=$(URL)"
	XCPPFLAGS += -DURL='"$(URL)"'

This is tedious and error-prone. Be a bit smarter by introducing
the DEFINES make variable which gets expanded twice to create the
command line options for m4 and cpp.