tweaks to build with clang-6 against GCC's math.h

First, switched a call to isnan() || isinf() to !isfinite(),
then switched all isfinite() to isfinite_(), which is
isfinite() with a couple warnings disabled.

That works around GNU's isfinite() that I'm picking up, which
looks something like

   return (sizeof(x) == sizeof(float )) ? isfinitef(x)
        : (sizeof(x) == sizeof(double)) ? isfinite (x)
        :                                 isfinitel(x);

Clang's warning about the isfinitef() and isfinitel() paths,
double->float demotion and double->long double promotion,
before compiling them away.  :/

Change-Id: I37c23e48707ac540c9c875ebc0bf30db905cd8cd
Reviewed-on: https://skia-review.googlesource.com/118680
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
3 files changed