/usr/include is now built on the fly, letting me merge include files from
multiple modules, which should be a win for organization later on.
binutils & gcc need to be recompiled.
limits.h shamelessly stolen from heat on #osdev, as gcc stopped providing me
with its own header. which was a hack in the first place
i've been wanting to do this for a while
the main benefit is that i can do stuff per-rule, so i can get rid of the
`@mkdir -p $(D)` i had everywhere, and in the near future i can make CC
targets automatically depend on a sysroot
dzwdz