Accept environment variables DEFAULT_*_TIDY_CHECKS

* Define DEFAULT_GLOBAL_TIDY_CHECKS/DEFAULT_EXTERNAL_VENDOR_TIDY_CHECKS
  only if it is not already defined.

Bug: http://b/27779618
Test: build with WITH_TIDY=1 and define those variables.
Change-Id: I201c9da3769d6ef658a75d4a1bf42ec6816d4069
1 file changed