`cabal build -v` prints one line to stderr

[lspitzner]

reproducable with both 1.22.6.0 and HEAD when building Cabal library itself. same symptom for other packages.

everything else is printed to stdout, so i guess this is not intended.

[23Skidoo]

What does it print to stderr?

[lspitzner]

/usr/bin/ar: creating dist/build/objs-18156/libHSCabal-1.23.0.0-5tmX9r4xC6b4kDzLiux47T.a

[ezyang]

Based on strace, this appears to be ar's fault. According to http://stackoverflow.com/questions/7027286/make-errors-ar-creating-lib-libmgr-a this can be suppressed by passing the -c flag. However, we need to make sure that this flag is respected by all ar we may want to invoke. We could also set up the subprocess call so that its stderr is pointed to our stdout, but then perhaps we should apply some uniform policy to how we treat the stderr of subprocesses twe call.