waf checking - fail on warnings?

[Hal Murray]

> A relatively quick search suggests mandatory=True as an argument.

That makes waf fail if the code chunk doesn't work.

I'm trying to make the code chunk not-work if it gets a warning, but then let 
waf put a comment in config.h to indicate that a feature test didn't work 
rather than put a #define to indicate that it did work.


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