this is for marking cmd options as paths relative to the
projects base dir. decorated paths can use tokens / macros
decorated paths are wrapped with "".  Final slashes are
honored. Slashes are platform specific.

Note that the working dir for custom cmd is undefined as
the current working dir will be different between xcode (wks.location)
and visual studio / make (prj.location.)  Changing the CWD isn't
a good default behavior (for reasons.)  This leads to prior
use of premake requiring alot of string concating / function calls
for path translations which make the cmd line difficult to read and maintain.
With path decorations, one can just make those relative paths with %[]
and everything should work.