cmake: CMAKE_CXX_FLAGS_DEBUG not evaluated correctly

CMAKE_CXX_FLAGS_DEBUG not evaluated correctly

Reproduce:

1) set some CMAKE_CXX_FLAGS_DEBUG flags in a cmake file, e.g. "-Og -D_DEBUG -g"

2) Run CMake over this cmake file

3) Assure that in the CMAKE_CXX_FLAGS_DEBUG  in the QT creator you wont find the set flags