Introduce expand_if_none_available to crosstool

This feature allows us to expand a flag_group when a build variable is not
available. This is helpful when migrating crosstools in a backward compatible
way (that works with released bazel as well as with bazel at HEAD).

RELNOTES: NONE.
PiperOrigin-RevId: 143955333
1 file changed
tree: d90dc630d6169587c9c7ccd78c684015857a54a1
  1. third_party/