[Bug #4953] Enable GTEST_HAS_ABSL by default when using bzlmod

[codelovingcat]

Summary

Enable GTEST_HAS_ABSL by default when using bzlmod/BCR.

Problem

When using bzlmod, abseil-cpp is an unconditional dependency, but GTEST_HAS_ABSL defaults to off.
This causes features like AbslStringify to fail, print incorrect values, or cause compilation errors.

Solution

• Added a new config_setting named bzlmod_enabled triggered by --enable_bzlmod=true.
• Updated //:gtest defines and deps select() statements to enable GTEST_HAS_ABSL=1 and link Abseil if either:
  • --define absl=1 (:has_absl)
  • bzlmod is enabled (:bzlmod_enabled)
• Updated docs/quickstart-bazel.md to note that Abseil support is enabled by default with bzlmod/BCR.

Backward Compatibility

• WORKSPACE users still need --define absl=1 to enable Abseil support.
• Behavior for existing workflows remains unchanged.

Testing

bazel test //... --enable_bzlmod=true

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