OSX/scons: make MACOSX_SDK_CHECK a local var is sconstruct and remove from env, also fix a longstanding misuse of MACOSX_SDK_CHECK in ceres, use MACOSX_SDK instead

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk@61201 954f8c5b-7b00-dc11-b283-0030488c597c