Teach ccmake to understand cache option strings.

This commit adds the ability to ccmake of cycling through cache options.
This uses the STRINGS property of the cache entry. The enter key will cycle
forward, and the right and left arrows will go up and down in the list.
Merge topic 'add_cache_options_to_ccmake' into next

4cc5c46 Teach ccmake to understand cache option strings.