no way to disable debug symbol generation with qmake

Without any /Z* option in CXX_FLAGS the generated Visual Studio project files don't contain a <DebugInformationFormat> tag.
VS uses /Zi as a default then, which is pretty bad on its own as that's even a lot slower than /Z7.
There's no way to explicitly disable it.

VCProjectWriter::write uses disabled by default, but VCXProjectWriter omits the tag. In the 5.1 version of toString(debugOption) you even added debugUnknown and debugDisabled cases, but with the same result.
I was able to hack it in by returning " " for debugDisabled instead of QString() which results in '<DebugInformationFormat> </DebugInformationFormat>'.