build some packages in another directory

The plan is to move all of it to build/ eventually, but
not everything builds currently.
Some don't build even without this change, and some fail
because of this change.
build Microwindows/Nano-X and libt1 in another directory too
update README
build fltk in another directory
README: Add instructions explaining git submodule steps
patches: new subdirectory for upstream patches not yet merged