Makefiles are pretty strange things.
There is a new wildcard target which will copy any .ogg file from
"assets/" into "out/assets". On it's own, it does nothing -- it only
knows how to create the output file by copying the input one.
To drive this, a list of output assets must be created and depended upon
by the main build target(s). I don't want to manually maintain an asset
manifest in a Makefile, so I've achived this by wildcard matching
anything in the "assets/" folder, then rewriting the prefix to be
"out/assets/". This list is in a variable, which is now part of the
dependency list for the main build target(s). All files will be
installed, but only when they are out-of-date. Excellent.
Variables that a package consumer might want to adjust should be placed
at the top of the file so they are immediately visible. Any constants
shall live below those (just the SRC folder, really).
