emulators/mupen64plus-core: Fix build with -fno-common

Add a partial patch from upstream to fix the build of
emulators/mupen64plus-core with -fno-common, which is the default with llvm

While here, point WWW: in pkg-descr to the current home of mupen64-plus.

MFH: 2020Q3 (implicit, -fno-common fixes, ok by joenum)