Removed setting of default values for variables which are never empty

Removed setting of default values for variables which are never empty
The standard autoconf variables like prefix, exec_prefix, ... are always set by
configure, so there is no need to provide explicit defaults in the Makefile.

The lines were introduced about a decade ago, perhaps there were some bugs in
ancient autoconfs, but today I can't think of a reason why this should be still
necessary.