Quick reference to CMake

if(<constant>)

True if the constant is 1, ON, YES, TRUE, Y, or a non-zero number. False if the constant is 0, OFF, NO, FALSE, N, IGNORE, NOTFOUND, the empty string, or ends in the suffix -NOTFOUND. Named boolean constants are case-insensitive. If the argument is not one of these constants, it is treated as a variable.

if(<variable>)

True if the variable is defined to a value that is not a false constant. False otherwise. (Note macro arguments are not variables.)

if(DEFINED <variable>)

True if the given variable is defined. It does not matter if the variable is true or false just if it has been set. (Note macro arguments are not variables.)