QtFlagHandlingHelpers.cmake overrides flags -On from the user

cmake /home/tjmaciei/src/qt/qt6/qtbase '-GNinja Multi-Config' '-DCMAKE_INSTALL_PREFIX=/home/tjmaciei/obj/qt/qt6/qtbase' --trace-expand '-DBUILD_TESTING=ON' '-DBUILD_WITH_PCH=OFF' '-DCMAKE_DEFAULT_BUILD_TYPE:STRING=Debug' '-DCMAKE_CONFIGURATION_TYPES:STRING=Release;RelWithDebInfo;Debug' '-DCMAKE_CXX_COMPILER:STRING=g++' '-DCMAKE_CXX_FLAGS:STRING=-march=skylake-avx512 -maes -mno-rdseed' '-DCMAKE_CXX_FLAGS_DEBUG:STRING=-O0 -g3' '-DCMAKE_CXX_FLAGS_RELEASE:STRING=-O3 -g1' '-DCMAKE_CXX_FLAGS_RELWITHDEBINFO:STRING=-O3 -g' '-DCMAKE_C_COMPILER:STRING=gcc' '-DCMAKE_C_FLAGS:STRING=-march=skylake-avx512 -maes -mno-rdseed' '-DCMAKE_C_FLAGS_DEBUG:STRING=-O0 -g' '-DCMAKE_C_FLAGS_RELEASE:STRING=-O3 -g1' '-DCMAKE_C_FLAGS_RELWITHDEBINFO:STRING=-O3 -g' '-DCMAKE_MAKE_PROGRAM=/usr/bin/ninja' '-DCMAKE_EXE_LINKER_FLAGS=-Wl,-O1 -Wl,--as-needed' '-DCMAKE_SHARED_LINKER_FLAGS=-Wl,-O1 -Wl,--as-needed' '-DFEATURE_dbus_linked=OFF' '-DFEATURE_developer_build=ON' '-DFEATURE_journald=ON' '-DFEATURE_libproxy=ON' '-DFEATURE_optimize_debug=OFF' '-DFEATURE_openssl_linked=ON' '-DFEATURE_reduce_relocations=ON' '-DFEATURE_sctp=ON' '-DFEATURE_system_sqlite=ON' '-DQT_LIBINFIX=.t' '-DQT_BUILD_EXAMPLES_BY_DEFAULT=OFF' '-DQT_BUILD_TESTS_BY_DEFAULT=OFF' '-DQT_QMAKE_TARGET_MKSPEC=linux-g++-optimised'

$ cmake -LA -N . | grep CMAKE_CXX_FLAGS_
CMAKE_CXX_FLAGS_DEBUG:STRING=-g3
CMAKE_CXX_FLAGS_MINSIZEREL:STRING=-DNDEBUG -Os
CMAKE_CXX_FLAGS_RELEASE:STRING=-g1 -O2
CMAKE_CXX_FLAGS_RELWITHDEBINFO:STRING=-g -O2