I think I found why are they missing: https://forums.libretro.com/t/missing-c ... list/14050
I indeed downloaded an MSVC build, because all downloads for Windows XP are MSVC2010 builds.You didn’t provide any information about your system to know what could be amiss, but I’m going to take a wild guess and assume you’ve installed one of the msvc builds, which only have access to a few of the cores. You’ll need to download the regular Windows 64-bit bundle/installer instead.
http://retroarch.com/?page=platforms
The tooltip also tells that:
So yes, it indeed limits my core choice. I guess that's what I get for still using Windows XP.NOTE: While this version will also work for Vista and up, we highly recommend you use the other Windows version instead, as your core choice will be more limited with this version.
The non-working Genesis Plus GX core was also an MSVC2010 build, while the working one is a MinGW build. (See the bug report.)
I guess RetroArch downloaded the MSVC2010 version because the installed RetroArch itself is a MSVC2010 build.