Auto merge of #2560 - alexcrichton:not-all-utf8, r=brson

Don't require all build script output to be utf-8

Build scripts often stream output of native build systems like cmake/make and
those aren't always guaranteed to produce utf-8 output. For example  German
MSVC cmake build has been reported to print non-utf-8 umlauts.

This commit instead only attempts to interpret each line as utf-8 rather than
the entire build script output. All non-utf-8 output is ignored.

Closes #2556