Update build docs that refer to old makefile.* files.

Updates some documentation, sparked from issue #3230.


I cannot test the build instructions myself, but assume they will work (what is there now doesn't work regardless).


The command

    find . -path '*/.git*' -prune -o -type f -exec grep makefile\\. {} \; -print

shows only 2 more files that reference the old makefile.* files:

            cd src; $(MAKE) -f makefile.unix bitcoind
            cd src; $(MAKE) -f makefile.unix clean
            cd src; $(MAKE) -f makefile.unix test_bitcoin
    ./contrib/debian/rules
          src/makefile.osx (Expat).
    ./contrib/debian/changelog