don't include FAKEROOT in Makefile variables

instead, prepend it when actually using them.
This makes the variables more useful for specifying on the make invocation,
as you don't have to repeat your FAKEROOT/DESTDIR for every variable you
want to set. Just like you can set 'lib' without specifying lib_prefix.

compare:
    make DESTDIR="${somevar}" prefix=/usr/local LIBDIR="${somevar}"/usr/local/lib96 MANDIR="${somevar}"/usr/share/man
to:
    make DESTDIR="${somevar}" prefix=/usr/local LIBDIR=/usr/local/lib96 MANDIR=/usr/share/man

Signed-off-by: Andrew G Morgan <morgan@kernel.org>