koga enhancements: defines, *.cxx and *.hpp as sources.

[dg1sbg]

This PR brings the following enhancements to koga:

1. Add source files with endings .cxx and .hpp as valid C++ source and header files.
2. Allow for #define statements being added in cscript files: (koga:defines "XYZ=ABC") adds -DXYZ=ABC to cflags.

The PR also includes a minor correction for a debug message saying that the locaton of an extension is taken from its repo definiton.

[yitzchak]

Thanks!