Don't redirect compiler stderr to stdout on Windows

It is not clear to me why this would be useful, except a slight performance enhancement because you need one less thread. This causes a problem in testproperties, which calls clm and passes stdout and stderr through. When testproperties is called by cleantest, cleantest expects only JSON test events on stdout, while stderr may be used for warnings. So if the compiler warnings are all on stderr and clm provides no extra output on stdout (as is the case after this commit), the cleantest user will get information about compilation errors/warnings while the tests continue to work.