Re-allow simple names for individual tool targets

Putting the generated executables into a subdirectory (in pull #74) meant that building individual targets like ```make -f posix rdmd``` stopped working and changed to (for example) ```make -f posix generated/linux/32/rdmd```. This restores the ability to build individual targets via ```make -f posix rdmd```, etc.

(This is a fixed version of #76.)