The default value is an empty string, which triggers the simple
transition. Passing null is not legal (and wasn't possible in
mkxp from Ruby side anyway).
Fixes#108.
The general rule I'm aiming for is to <> include
system wide / installed paths / generally everything
that's outside the git managed source tree (this means
mruby paths too!), and "" include everything else,
ie. local mkxp headers.
The only current exception are the mri headers, which
all have './' at their front as to not clash with
system wide ruby headers. I'm leaving them be for now
until I can come up with a better general solution.
Jonas Kulla