Fix deps wrongly using separate_builds_apple
requested to merge freyalupen/krita-deps-management:freyalupen/remove-separate-builds-apple into master
eigen3, expat, exiv2, and fftw3 don't use
kis_ExternalProject_Add_with_separate_builds_apple
according to Krita's 3rdparty folder, so it seems like a mistake. (Especially the one under an ifdef for WIN32?)
On a related note, that macro hasn't been implemented here, so anything using it will fail on macOS.