d: gdc: Implement proper dependency gen support
d: gdc: Implement proper dependency gen support

Older versions of GDC had broken support for the -MQ flag (resulting in
a compiler error). New versions of GDC have this issue fixed (some older
versions received a backport). Starting with GCC 7.1 we can assume
dependency generation to always work reliably.

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