Remember which Visual Studio backend was auto-detected
Remember which Visual Studio backend was auto-detected

This fixes the REGEN build target since VSINSTALLDIR does not get set by
Visual Studio when building targets.

Fixes #2848.

QuLogic/meson,MathieuDuponchelle/meson,becm/meson,QuLogic/meson,jpakkane/meson,thiblahute/meson,thiblahute/meson,pexip/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,jpakkane/meson,thiblahute/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,pexip/meson,jeandet/meson,mesonbuild/meson,becm/meson,jeandet/meson,pexip/meson,thiblahute/meson,becm/meson,jpakkane/meson,mesonbuild/meson,MathieuDuponchelle/meson,jeandet/meson,becm/meson,QuLogic/meson,mesonbuild/meson,jeandet/meson,MathieuDuponchelle/meson,QuLogic/meson,thiblahute/meson,QuLogic/meson,pexip/meson,QuLogic/meson,becm/meson,becm/meson,MathieuDuponchelle/meson,becm/meson,jeandet/meson,MathieuDuponchelle/meson,QuLogic/meson,pexip/meson,jpakkane/meson,QuLogic/meson,jpakkane/meson,mesonbuild/meson,jpakkane/meson,thiblahute/meson,MathieuDuponchelle/meson,becm/meson,mesonbuild/meson,pexip/meson,mesonbuild/meson,MathieuDuponchelle/meson,jpakkane/meson,pexip/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,thiblahute/meson,jeandet/meson,pexip/meson,jpakkane/meson,mesonbuild/meson,jeandet/meson,becm/meson,jeandet/meson,thiblahute/meson,QuLogic/meson,thiblahute/meson,becm/meson,jeandet/meson