Clean up and fix minor bugs in DEBUG/GOGCFLAGS handling
Clean up and fix minor bugs in DEBUG/GOGCFLAGS handling

* Use “override GOGCFLAGS+=” so that (make GOGCFLAGS=… DEBUG=1)
  does not ignore the appending to GOGCFLAGS
* Move quoting of -gcflags from the variable to its use,
  so that (make GOGCFLAGS=… DEBUG=1) is correctly quoted
* Now that GOGCFLAGS and DEBUG are both handled correctly when
  completely empty, simplify by dropping the DEBUG!=1 branch.
* Beautify the command line by not using DEBUG= if DEBUG is unset.

jwhonce/skopeo,mtrmac/skopeo,mikebrow/skopeo,mikebrow/skopeo,mtrmac/skopeo,jwhonce/skopeo,projectatomic/skopeo,projectatomic/skopeo,mtrmac/skopeo