CMake: do not expand variables beforehand
Fixes build when FindOpenGL is disabled, producing a syntax error: "Function invoked with incorrect arguments for function named: ADD_FEATURE_INFO"
See also: https://invent.kde.org/frameworks/plasma-framework/-/merge_requests/267
Downstream report: https://bugs.gentoo.org/809815
Signed-off-by: Andreas Sturmlechner asturm@gentoo.org