Auto merge of #3363 - alexcrichton:libcmt, r=alexcrichton

Compile statically against the MSVC CRT

This updates our AppVeyor builds to compile with `-Ctarget-feature=+crt-static`
to help Cargo be a bit more portable and not rely on the MSVC redistributable
artifacts. Over time this may even let us converge on only releasing one build
of Cargo and just pairing that with all Windows toolchains...