...It's a thing that isn't, at least not by default. If you use MinGW on Windows and want to build 64-bit executables with SDL you'll need to go to a site like http://www.drangon.org/mingw/ to get pre-compiled dev libs or something since SDL's site only has 32-bit libs. More info as progress warrants...

Isn't there a way to link to MSVC-compiled dlls from mingw? They do provide (and it's pretty easy to make yourself) 64-bit MSVC libraries.

I'm not familiar with the MSVC way of doing things; I just skipped ahead to getting SDL 2.0.0 dev libs instead and my code compiles on 64-bit and runs successfully

I think that there is something called 'dlltool' or something like that that mingw comes with that can turn MSVC libraries+lib or exp files (symbols external only by including the lib or exp file) into Mingw style (gcc style, all symbols external) libraries and the other way around, too. Not properly making SDL using MingW, but allowing it to be easily used with MingW.