math: add portability warnings for classification macros

math: add portability warnings for classification macros

For interfaces which are defined to exist only as macros, implement
the proper magic needed to warn if the corresponding gnulib module was
not in use.  This solution avoids promoting arguments unnecessarily,
to allow reuse in case gnulib ever implements fpclassify, since
fpclassify(1e-40f) is different than fpclassify(1e-40).

* modules/math (Depends-on): Add warn-on-use.
(Makefile.am): Provide new substitutions.
* m4/math_h.m4 (gl_MATH_H): Require inline.
* lib/math.in.h (_GL_WARN_REAL_FLOATING_DECL)
(_GL_WARN_REAL_FLOATING_IMPL): New helper macros.
(isfinite, isinf, isnan, signbit) [GNULIB_POSIXCHECK]: Use them to
implement warnings.

Signed-off-by: Eric Blake <ebb9@byu.net>