CMake: do not expand variables beforehand

Fixes:
```
CMake Error at webenginepart/src/CMakeLists.txt:113 (if):
  if given arguments:

    "(" "NOT" "0" ")" "AND"

  Unknown arguments specified
```

See also:
https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/267



Signed-off-by: Andreas Sturmlechner <asturm@gentoo.org>