The line numbers in preproc's errors involving strings or INCBINs were
incorrect before this change because all the lines from '#include'd
files incremented the line number that would be reported.
'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'.
