Fixed Makefile BUILD_FLAGS for darwin

[mcwienczek]

Changed makefile to use bazel darwin build flag

fixes #1581

Release note:

`make` command failed with bazel 0.7 because of unsupported build flag `k8`. Looks like BUILD_FLAGS should be darwin on macos, not k8 anymore. 

[istio-testing]

Hi @mcwienczek. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

I understand the commands that are listed here.

Instructions for interacting with me using PR comments are available [here](https://github.com/kubernetes/community/blob/master/contributors/devel/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository. I understand the commands that are listed [here](https://github.com/kubernetes/test-infra/blob/master/commands.md).

[istio-merge-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: We suggest the following additional approver: kyessenov

Assign the PR to them by writing /assign @kyessenov in a comment when ready.

The full list of commands accepted by this bot can be found here.

Needs approval from an approver in each of these OWNERS Files: - **[OWNERS](https://github.com/istio/pilot/blob/master/OWNERS)** You can indicate your approval by writing `/approve` in a comment You can cancel your approval by writing `/approve cancel` in a comment

[codecov[bot]]

Codecov Report

Merging #1624 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1624   +/-   ##
=======================================
  Coverage   82.78%   82.78%           
=======================================
  Files          52       52           
  Lines        6423     6423           
=======================================
  Hits         5317     5317           
  Misses        901      901           
  Partials      205      205

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 6f20fc3...bb7cc8d. Read the comment docs.

[istio-testing]

@codecov[bot]: you can't request testing unless you are a istio member.

In response to [this](https://github.com/istio/pilot/pull/1624#issuecomment-339426580): ># [Codecov](https://codecov.io/gh/istio/pilot/pull/1624?src=pr&el=h1) Report >> Merging [#1624](https://codecov.io/gh/istio/pilot/pull/1624?src=pr&el=desc) into [master](https://codecov.io/gh/istio/pilot/commit/6f20fc31474dae9a944d47a364ded3bb3439c477?src=pr&el=desc) will **not change** coverage. >> The diff coverage is `n/a`. > >[](https://codecov.io/gh/istio/pilot/pull/1624?src=pr&el=tree) > >```diff >@@ Coverage Diff @@ >## master #1624 +/- ## >======================================= > Coverage 82.78% 82.78% >======================================= > Files 52 52 > Lines 6423 6423 >======================================= > Hits 5317 5317 > Misses 901 901 > Partials 205 205 >``` > > > >------ > >[Continue to review full report at Codecov](https://codecov.io/gh/istio/pilot/pull/1624?src=pr&el=continue). >> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta) >> `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` >> Powered by [Codecov](https://codecov.io/gh/istio/pilot/pull/1624?src=pr&el=footer). Last update [6f20fc3...e7a3da3](https://codecov.io/gh/istio/pilot/pull/1624?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). > Instructions for interacting with me using PR comments are available [here](https://github.com/kubernetes/community/blob/master/contributors/devel/pull-requests.md). If you have questions or suggestions related to my behavior, please file an issue against the [kubernetes/test-infra](https://github.com/kubernetes/test-infra/issues/new?title=Prow%20issue:) repository.

[rshriram]

/ok-to-test

[rshriram]

/test pilot-e2e-smoketest