Auto merge of #2006 - alexcrichton:no-dylibs, r=brson

Currently it's not possible to inform Cargo about target-specific crate types,
so generating a hard error whenever a dylib is seen for a target that can't
produce dylibs is a little heavy. This commit disables management of the output
dylib (which won't actually exists) and relies on the compiler to print a
warning in the case of a dylib output on a non-dylib target.