Qt6 CMake: RDSEED test fails with --qt-cflags-not-found

-- Performing SIMD Test RDSEED instruction
-- Performing SIMD Test RDSEED instruction - Failed

$ cd config.tests/x86_simd_rdseed
$ ninja
[1/2] Building CXX object CMakeFiles/SimdTestrdseed.dir/main.cpp.o
FAILED: CMakeFiles/SimdTestrdseed.dir/main.cpp.o 
/home/tjmaciei/bin/g++ -DQT_COMPILER_SUPPORTS_RDSEED  -g -O3 -feliminate-unused-debug-types  -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=32 -Wformat -Wformat-security -Wl,--copy-dt-needed-entries -m64  -fasynchronous-unwind-tables -Wp,-D_REENTRANT -ftree-loop-distribute-patterns -Wl,-z -Wl,now -Wl,-z -Wl,relro -malign-data=abi -fno-semantic-interposition -ftree-vectorize -ftree-loop-vectorize  -fvisibility-inlines-hidden --qt-cflags-not-found -std=gnu++17 -o CMakeFiles/SimdTestrdseed.dir/main.cpp.o -c /home/tjmaciei/src/qt/qt6/qtbase/config.tests/x86_simd/main.cpp
g++: error: unrecognized command-line option ‘--qt-cflags-not-found’
ninja: build stopped: subcommand failed.