Detect Xcode version gracefully (#1192)
Detect Xcode version gracefully (#1192)

`is_xcode_at_least_version` calls `fail()` when it cannot detect an Xcode
version. This happens on non-macOS platforms. As this function is
called in Analysis Phase, it makes build fail early and potentially
prevents the usage of `target_compatible_with`.

Fixes #1189
bazelbuild/rules_apple,bazelbuild/rules_apple,bazelbuild/rules_apple,bazelbuild/rules_apple