Migrate from MinGW to mingw-w64

Important: depends on !1 (merged) to be merged first but can be tested independently

Windows x86-64 builds are successful but yet untested (same as linux x86-64)

patches/SDL_sysevents.c.patch

---- SDL_sysevents.c	2009-04-17 17:18:07.000000000 +0200
-+++ SDL_sysevents.c.new	2009-04-17 17:14:41.000000000 +0200
-@@ -799,7 +799,7 @@
- #define CS_BYTEALIGNCLIENT	0
- #endif
- 	if ( ! name && ! SDL_Appname ) {
--		name = "SDL_app";
-+		name = "BOINC_app";
- 		SDL_Appstyle = CS_BYTEALIGNCLIENT;
- 		SDL_Instance = hInst ? hInst : SDL_GetModuleHandle();
- 	}