'INCGFX' is like 'INCBIN' except that it specifies the arguments to pass
to 'gbagfx' which alleviates the user-unfriendliness caused by catch-all
rules.
Specifically, users frequently forget to add to 'spritesheet_rules.mk'
when adding object event graphics, and then the built '.4bpp' file is
not invalidated when the add their rule so they have to 'touch' the
source file or similar.
The built artifacts are placed in 'build/assets'.
Merge C_ASM_DEP and DATA_ASM_DEP, NODEP and SCAN_DEPS can be overridden, add --no-print-directory to MAKEFLAGS in make_tools.mk (also removed some unused variables), add newline to help message in preproc.