Use AppVeyor for Windows testing
Skip EM specs on Windows
Update for MinGW-x64
Add support/libmysql.def
Generate interface library libmysql.a from MySQL Connector/C libmysql.lib and the pre-generated libmysql.def included here
Use env var RUBY_MYSQL2_LIBMYSQL_DLL to find libmysql.dll out of path
Vendor libmysql.dll by default on Windows. Use rpath only on Unix platforms.