restructure building process

Right now, we do three build passes: one to build the original tree
(full) build, one to build the patch tree (diff build), then one to
rebuild original objects that where changed by the patch (diff build).

This is going to be a problem when we try to support (near) full tree
rebuilds due to changes in commonly included header files.

This commit changes the build process to intercept calls to gcc by make
using the CROSS_COMPILE environment variable and, during the patched
build phase, copies the original object for any object that is about to
rebuilt due to a change.

This reduces the number of build passes to the minimum possible: 2.

Signed-off-by: Seth Jennings <sjenning@redhat.com>