Xcode 12.4, isdigit() failure with C Flags

I have a simple project that I built to track down a bizarre problem.

In my applicationDidFinishLaunching:, I call isdigit().

If I add a C Flag such as -DBUILD_RETAIL or a preprocessor macro such as BUILD_RETAIL=1, then I get:

Implicitly declaring library function 'isdigit' with type 'int (int)'

This happens even if I add #import <ctype.h> to the .m file.

I am on 10.15.7 with Xcode 12.4
Xcode 12.4, isdigit() failure with C Flags
 
 
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