* Problem: No CI for mingw64

Adding CI support for mingw64 to appveyor.yml

Reason: To help address the issue:
  Several checks fail under 64-bit MinGW on Windows when running make check #3185

* Problem: Docs do not build with mingw64, dll already in right place

Turning off WITH_DOC for mingw64
Removing copy command for mingw64 and cygwin64 (it now seems to be directly built in "bin" directoy)