[Issue 17787] Add a BetterC predefined version so libraries can adapt

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=17787

--- Comment #6 from github-bugzilla at puremagic.com ---
Commit pushed to stable at https://github.com/dlang/dmd

https://github.com/dlang/dmd/commit/ca27ec4de2f7054aab3ae1fd5aae21639952bf26
Merge pull request #7132 from WalterBright/D_betterC

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