redesign and rewrite Makefiles

Bootstrapping is no longer needed, there is only one (minimal) root
Makefile left.  Admin tasks are new in make/mk.admin while global
variables and common targets were moved to make/mk.common.

Global variables (i.e. variables in make/mk.common) were reduced to a
minimum.  Subdir makefiles now follow a standard:  generated content is
grouped in OUT_[A-Z] variables.  Content that needs to be installed and
uninstalled belongs into INST (artificial suffixes indicate *how* files
should be installed).  Finally, flags, dependencies and build
instructions are defined.

Noteworthy targets:

    $ make build
    $ make clean
    $ make install
    $ make uninstall
    # default targets that work in rootdir or any subdir

    $ make dist
    # make distribution archive

    $ make configure
    # creates configure script (ex. bootstrap)

    $ make distclean
    # cleans generated content by autoconf and make/mk.admin

    $ make .gitignore
    # generates a complete (!) .gitignore