CMP0012
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if() recognizes numbers and boolean constants.

In CMake versions 2.6.4 and lower the if() command implicitly
dereferenced arguments corresponding to variables, even those named
like numbers or boolean constants, except for 0 and 1.  Numbers and
boolean constants such as true, false, yes, no, on, off, y, n,
notfound, ignore (all case insensitive) were recognized in some cases
but not all.  For example, the code "if(TRUE)" might have evaluated as
false.  Numbers such as 2 were recognized only in boolean expressions
like "if(NOT 2)" (leading to false) but not as a single-argument like
"if(2)" (also leading to false).  Later versions of CMake prefer to
treat numbers and boolean constants literally, so they should not be
used as variable names.

The OLD behavior for this policy is to implicitly dereference
variables named like numbers and boolean constants.  The NEW behavior
for this policy is to recognize numbers and boolean constants without
dereferencing variables with such names.

This policy was introduced in CMake version 2.8.0.  CMake version
|release| warns when the policy is not set and uses OLD behavior.  Use
the cmake_policy command to set it to OLD or NEW explicitly.