README: clean up some whitespace

Remove $ prefix, use tab consistently.

Signed-off-by: Sage Weil <sage@inktank.com>
README: move package dependencies into separate files

Sorted, with a short, simple command to paste.  Avoid the duplication of
the lists.

Signed-off-by: Sage Weil <sage@inktank.com>
debian: improve package dep list

This is based on the Build-Depends in debian/control.

Signed-off-by: Sage Weil <sage@inktank.com>
Makefile: make install-deps rule

This only works if autoconf is happy, but it's better than nothing.

Fixes: #8259
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1847 from ceph/wip-8259

README, Makefile: make install-deps easier

Reviewed-by: Loic Dachary <loic@dachary.org>