[issue15495] enable type truncation warnings for gcc builds

[Jeremy Kloth]

Jeremy Kloth added the comment:

> I'm getting more than thousand warnings:
>
> $ CFLAGS="-Wconversion -Wno-sign-conversion" ./configure
> $ LC_ALL=C make -s -j8 2>&1 | tee log
> $ grep Wconversion log | wc -l
> 1163

My Win64 buildbot currentlyhas 544 conversion warnings, but it seems
something has recently changed with OpenSSL thus causing over 2100
total warnings.

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nosy: +jeremy.kloth

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<http://bugs.python.org/issue15495>
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