Add mingw-w64 builds to appveyor.yml

Currently the cygwin autoconf builds only test builds of cygwin-native
binaries, and not mingw-w64 pure windows binaries. The resultant
cygyaml-0-2.dll has a runtime dep on the cygwin1.dll which is generally
not appropriate for the purpose of distribution. Building with mingw-w64
produces libraries with no dependency on the cygwin runtime.

Signed-off-by: Marty E. Plummer <hanetzer@protonmail.com>