Allow INCLUDEDIRS to accept absolute paths

[jake-low]

The docs claim (and my own testing has verified) that INCLUDEDIRS accepts only relative paths.

It'd be neat if you could define variables with mk_declare that listed a set of related header files, and then use those variables in INCLUDEDIRS parameters in other MakeKitBuild scripts, thereby reducing the amount of redundant listing of header directories in projects where each directory contains a MakeKitBuild file. What currently prevents this is that INCLUDEDIRS must take relative paths (which wouldn't resolve consistently across several MakeKitBuild files). If INCLUDEDIRS were able to accept absolute paths, you could use mk_resolve_target and mk_declare to build a list of include directories, and reuse it everywhere.