Build: GNUmakefile: tweak default target

Previously, the default target would run autogen.sh if configure didn't exist,
otherwise it would run the core target.

Now, it behaves similarly whether autogen.sh has been run yet or not: if
configure doesn't exist, it runs autogen.sh; if Makefile doesn't exist, it
runs configure; then it runs the core target.

This allows for a similar experience running "make" whether in a raw or
configured directory, and handles the case where autogen.sh has been run
but configure hasn't (e.g. after "make distclean").