Add tribits_invalid_value() function for else statements for enums

[bartlettroscoe]

Description

There is a lot of code in TriBITS that passed strings as enums like:

if (<varName> STREQUAL "<valueA>")
  ...
elseif (<varName> STREQUAL "<valueB>")
  ...
elseif (<varName> STREQUAL "<valueC>")
  ...
else()
  message(FATAL_ERROR "Error, the value of <varName>='${<varName>}' is not in the allowed list ...")
endif()

It may help to replace this with a function like:

else()
  tribits_invalid_value(<varName> [<listOfValidValues>])
endif()

where the list of valid values var name <listOfValidValues> could be empty.

NOTE: Something like this was suggested by @KyleFromKitware in #560 (review).