fix(cmake): do not disable feature if option GOOGLE_CLOUD_CPP_ENABLE_* does not exist

[GaetanLepage]

GOOGLE_CLOUD_CPP_ENABLE features which do not have a corresponding GOOGLE_CPP_CPP_ENABLE_<name> feature are getting filtered out wrongfully.

Indeed, google_cloud_cpp_enable_cleanup will disable a feature when NOT "${${feature_flag}}".
This statement will be true in two cases:

1. When GOOGLE_CPP_CPP_ENABLE_<name> is explicitly disabled (which is the intended behavior)
2. When GOOGLE_CPP_CPP_ENABLE_<name> does not exist (unintended)

Indeed, this leads to universe_domain being filtered out as GOOGLE_CPP_CPP_ENABLE_UNIVERSE_DOMAIN was not declared in google_cloud_cpp_define_legacy_feature_options.

This PR fixes the google_cloud_cpp_enable_cleanup behavior by disabling a feature when both DEFINED "${${feature_flag}}" and NOT "${${feature_flag}}".

Another alternative would be to declare GOOGLE_CPP_CPP_ENABLE_UNIVERSE_DOMAIN, but I understood that these options were "legacy" and that their usage is discouraged.

[gemini-code-assist]

Summary of Changes

Hello @GaetanLepage, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a bug in the CMake build system's feature management, specifically within the google_cloud_cpp_enable_cleanup macro. Previously, features were unintentionally disabled if their dedicated GOOGLE_CLOUD_CPP_ENABLE_* option was not defined, rather than only when explicitly set to false. The change introduces a DEFINED check, ensuring that features are now correctly enabled by default unless explicitly opted out, thereby preventing erroneous feature removals like the one observed with universe_domain.

Highlights

• Incorrect Feature Disabling: The google_cloud_cpp_enable_cleanup macro was incorrectly disabling features when their corresponding GOOGLE_CLOUD_CPP_ENABLE_* CMake option was undefined, rather than explicitly set to false.
• Impact on 'universe_domain': This bug specifically led to features like universe_domain being filtered out because GOOGLE_CLOUD_CPP_ENABLE_UNIVERSE_DOMAIN was not declared, causing unintended behavior.
• Conditional Logic Fix: The fix introduces a DEFINED check within the macro's conditional logic, ensuring that a feature is only disabled if its enable flag is both explicitly defined and set to a false value.

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[gemini-code-assist]

Code Review

This pull request addresses a bug in the CMake build logic where a feature could be incorrectly disabled. The issue arose when a legacy GOOGLE_CLOUD_CPP_ENABLE_<feature> option was not defined, as this was treated as a 'false' value, leading to the feature being filtered out. The fix introduces a check to verify that the legacy option is defined before its value is evaluated. This change is correct and makes the build configuration more robust by preventing features from being unintentionally disabled.