Fix a warning.
Make rawSystemStdInOut more verbose on error.
Make it possible to build empty libraries again.

Restores old behaviour broken by @igfoo in
8395c5db800284c9e2b54b5c45b94c4a3bfdfcf2. Fixes #1241.
Add a test for #1241.
Merge pull request #1249 from 23Skidoo/issue1241

 Make it possible to build empty libraries again.