Fixes for Legion sub-build

[jjwilke]

Fixes for two subtle issues around Legion. These issues only occur when re-configuring, not on the initial configure.

• Legion_DIR gets set to -NOTFOUND on the initial configure, which was then triggering a REQUIRED on re-configure since it was only checking DEFINED rather than the Boolean-ness. The changes ensure that re-configuring does not trigger an error.
• Legion_MAX_DIM and Legion_MAX_FIELDS ended up with different values after re-configure due to subtleties with set() and option().