Allow source lists to carry extra attributes

Use it to move the transform_target parameter into the list itself.

Fix the non-OpenGL build, which should have had a transform_target
parameter and did not.
Chain objects_common lookup to simplify users
Chain objects_editor property
Add Win32PlatformSettings.__init__ to fix SConstruct for Windows
Use program.srcdir, not self.srcdir for Win32PlatformSettings
Add support for cross-compilation
Fix path to .rc file for Windows build
Move lazy object construction to a separate class
Fix longstanding bug preventing build of both Windows binaries at once

SCons rejected building both Windows targets due to conflicting
instructions on how to compile messagebox.c.  Restructure the lookup so
that only one set of instructions is issued.
Move */main/vers_id.c -> similar/main/vers_id.c
Run banner() sooner
Move RPi defaults up so they can be shown without a program
Move path defaults up so they can be shown without a program