[CMake] evaluating a simple boolean expression?

[Brandon J. Van Every]

How do I evaluate a simple boolean expression in CMake?  I've tried {} 
and ${} and they don't work for expressions, only for variable 
substitutions.  () and $() give errors.  [] and $[] appear to be absent 
from the language.  I've Googled all over the place and can't find any 
documentation.  I'm looking for something like an EVAL() function.  Why 
do I need an evaluator?  I get tired of writing my boolean conditionals 
twice:

SET(HAVE_EXTANT_CHICKEN true)
IF(${EXTANT_CHICKEN} STREQUAL "EXTANT_CHICKEN-NOTFOUND")
  SET(HAVE_EXTANT_CHICKEN false)
ENDIF(${EXTANT_CHICKEN} STREQUAL "EXTANT_CHICKEN-NOTFOUND")
IF(HAVE_EXTANT_CHICKEN)
  blah blah blah...
ELSE(HAVE_EXTANT_CHICKEN)
  bleh bleh bleh...
ENDIF(HAVE_EXTANT_CHICKEN)

I would rather isolate my conditional and only edit it in 1 place.  
Particularly when conditionals get longer.  Something like:

SET(HAVE_EXTANT_CHICKEN {${EXTANT_CHICKEN} STREQUAL 
"EXTANT_CHICKEN-NOTFOUND"})
IF(HAVE_EXTANT_CHICKEN)
  blah blah blah...
ELSE(HAVE_EXTANT_CHICKEN)
  bleh bleh bleh...
ENDIF(HAVE_EXTANT_CHICKEN)


Cheers,
Brandon Van Every