CMake: Simplify if() conditions on check result variables

Remove use of an old hack that takes advantage of the auto-dereference
behavior of the if() command to detect if a variable is defined.  The
hack has the form:

 if("${VAR} MATCHES "^${VAR}$")

where "${VAR}" is a macro argument reference.  Use if(DEFINED) instead.
This also avoids warnings for CMake Policy CMP0054 in CMake 3.1.