Use isinf builtin for clang, for performance.

When compiling with clang glibc's definition of isinf() ends up
leading to and external libc function call. That's because there was a
bug in the builtin in an old gcc version, and clang claims
compatibility with an older version.  That causes clang to be
measurably slower for floating point heavy workloads than gcc.

To fix simply redirect isinf when using clang and clang confirms it
has __builtin_isinf().

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