We currently have three macros (as in "functions", not just simple strings) that are assigned in spec.gmk. This is in contrast with the rest of the values assigned in spec.gmk, and it is surprising and is forcing an odd syntax. Furthermore, these macros are named as constants, and not as we normally name functions/macros.

We should move them to normal .gmk code, and rename them.

The three macros are `SET_SHARED_LIBRARY_ORIGIN`, `SET_EXECUTABLE_ORIGIN` and `SET_SHARED_LIBRARY_NAME`.