Replace obsolete isinff(...)

[fxing-qnx]

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Fixes # N/A

Proposed changes: Replace isinff(...) with a combination of isinf(...) and signbit(...)

Benchmark results: No changes in performance.

Test suite changes: N/A

Almost the same to https://github.com/ebassi/graphene/pull/188, but more aggressive. isinff(...) is completely remove from the source tree. Reason: Mentioned in PR188, isinff(...) is both obsolete and non-standard. To maximize portability and suggested by GNU, we should have it replaced.

PS, Sorry due to our process we need to always fork under tags even if a tag only contains version changes. I have a rebased branch ready for merge now.

Edit: Fix #278