Add in something the builder can override to augment the GO builds

I've been looking at reasons packagers are not building the Go binaries
and found this with respect to RPMs:

  https://github.com/rpm-software-management/rpm/issues/367

There has been no easy way to inject the otherwise unneeded workaround:
-ldflags=-linkmode=external for building (which, strangely, generates
some sort of warning and gratuitously links glibc to an otherwise
static build), but seems to work.

Until RPM supports Go's native '.note.go.buildid', and RPM requires
'.note.gnu.build-id' on binaries, I guess this can work around it:

   GO_BUILD_FLAGS='-ldflags=-linkmode=external'

Signed-off-by: Andrew G. Morgan <morgan@kernel.org>
2 files changed