Two updates suggested by Paul Koning:
- Enable building multiple simulator targets using a comma-separated list,
e.g.: pdp8,pdp11,3b2
- Unconditionally clean CMake's cache each time the script is invoked.
This eliminates confusing CMake configuration messages when CMake
changes or updates, and CMake policies change.
The most recent policy change was IMPORT_LOCATION, which is now
mandatory for interface libraries. An old CMake configuration cache
doesn't reflect the import library IMPORT_LOCATION property, which
caused confusing warning messages. The cure is removing the old cache
and reconfiguring.
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