Faster make v2

[dgageot]

This solves multiple slowness issues with the current Makefile:

• Some variables are eagerly evaluated even if they are not used by the current target.
• make generate-statik should make its inputs and outputs explicit so that we can avoid the cost of regenerating

The net result is that make and make install will be faster to find out that nothing needs to be rebuilt and will be faster to rebuild changes to the Go code.

@nkubala FYI, this is a fixed version of #3706

[codecov]

Codecov Report

Merging #3724 into master will not change coverage.
The diff coverage is n/a.

[tejal29]

After going through http://www.gnu.org/software/make/manual/make.html#Flavors,
I feel simply expanded variables and make more sense in this Makefile.

Is there a reason why you changed to "recursively expanded" ?

[tejal29]

Testing notes:
Make is definitely faster

tejaldesai@@skaffold (faster-make-v2)$ time make
hack/generate-statik.sh
Collecting licenses
Collecting schemas
GOOS=linux GOARCH=amd64 CGO_ENABLED=1 go build -tags "osusergo netgo static_build release" -ldflags " -X github.com/GoogleContainerTools/skaffold/pkg/skaffold/version.version=v1.4.0-11-g15c669938-dirty -X github.com/GoogleContainerTools/skaffold/pkg/skaffold/version.buildDate=2020-02-27T15:22:04Z -X github.com/GoogleContainerTools/skaffold/pkg/skaffold/version.gitCommit=15c6699381e1dca6e5f1faa939df350576912668 -X github.com/GoogleContainerTools/skaffold/pkg/skaffold/version.gitTreeState=dirty -s -w  -extldflags \"-static\"" -o out/skaffold github.com/GoogleContainerTools/skaffold/cmd/skaffold

real	0m26.161s
user	0m48.223s
sys	0m1.393s
tejaldesai@@skaffold (faster-make-v2)$ time make
make: 'out/skaffold' is up to date.

real	0m0.356s
user	0m0.290s
sys	0m0.071s
tejaldesai@@skaffold (faster-make-v2)$ 

Tested the cloudbuild.yaml.

$git clean -fdx cmd/
Removing cmd/skaffold/app/cmd/statik/statik.go

$gcloud builds submit --config deploy/cloudbuild.yaml --substitutions=_RELEASE_BUCKET=tejal-test,COMMIT_SHA=$(git rev-parse HEAD) --project tejal-test
...
Success

TAG_NAME=vtest3724 gcloud builds submit --config deploy/cloudbuild-release.yaml --substitutions=_RELEASE_BUCKET=tejal-test --project tejal-test
..

SUCCESS

[tejal29]

After testing described in #3724 (comment), LGTM!

Thank you!