There are now C99 functions that the printf items want to use that may
not be necessarily supported by the math.h that is shipped by the
compiler, such as signbit(), isinf(), isnan() and isfinite() and their
double, long and float counterparts.

This checks for whether these functions are provided by the math.h
shipped by the compiler, and builds the gnulib implementations of them
if they cannot be found.  Currently no attempt is made to check whether
these, if available from the compiler's math.h, are compliant with the
specs.