Use MACOSX_DEPLOYMENT_TARGET instead of SDKROOT.

Use MACOSX_DEPLOYMENT_TARGET instead of SDKROOT.

MACOSX_DEPLOYMENT_TARGET is the minimum unconditionally supported OS,
which should just be 10.6 for us until Chrome changes.  SDKROOT is the
maximum conditionally supported OS, which defaults sanely to whatever's
the latest SDK the machine has, so we don't need to mention it.

BUG=skia:
R=bungeman@google.com

Author: mtklein@google.com

Review URL: https://codereview.chromium.org/137793010

git-svn-id: http://skia.googlecode.com/svn/trunk@13102 2bbb7eff-a529-9590-31e7-b0007b416f81