ext: Update fputils to rev 6a47fd8358

This patch updates fputils to the latest revision (6a47fd8358) from
the upstream repository (github.com/andysan/fputils). Most notably,
this includes changes that export a limited set of 64-bit float
manipulation and avoids a warning about unused 64-bit floats in clang.

ext: Fix fputils compiler flags to ensure ISO C99

The fp code relies on C99, and depending on gcc version, the default
is to use c89. This patch adds -std=c99 when using gcc to ensure the
code is compiled in ISO C99 mode.

ext: Include libfputils

This changeset includes libfputils from revision bbf0d61d75. This
library can be used to convert to and from 80-bit floats and query the
type of an 80-bit float, which is needed to support the x87 FPU.