[Cmake] Issues and new ideas

[Amitha Perera]

> 2. A third state will be added as a possible cache value.
> NOTFOUND, IGNORE, or VALUE.

There'll need to be an corresponding extension to the IF command. I
suggest the third state be called "UNSET" or something like that:
  IF(HAS_LIB)        - succeeds if HAS_LIB is not "UNSET" and is not "NO" 
                       ("NOTFOUND", "FALSE", "0" and other negatives)
  IF(NOT HAS_LIB)    - not( above )
  IF(SET HAS_LIB)    - succeeds if HAS_LIB is not "UNSET"
  IF(NOTSET HAS_LIB) - for convenience

Using the variable:

IF(HAS_LIB)
   do something with it
ENDIF(HAS_LIB)

In the FindLib module:

IF(NOTSET HAS_LIB)
  SET( HAS_LIB "NO" CACHE )
  FIND_LIBRARY(LIB_PATH...)
  IF(LIB_PATH)
    SET( HAS_LIB "YES" CACHE )
  ENDIF(LIB_PATH)
ENDIF(NOTSET HAS_LIB)