waf checking - fail on warnings?

[Hal Murray]

Matthew.Selsky at twosigma.com said:
> And you want to pass "-Werror" (I'm not certain how off the top of my head)
> to the compiler so that warnings are fatal. waf sees the compiler exit zero
> with or without warnings, so they look the same. 

I put it into ctx.env.CFLAGS, then restored CFLAGS back to the original after 
running the test.


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