* lib/isnanf-nolibm.h (isnanf): Use the GCC built-in __builtin_isnan,
not __builtin_isnanf. Also on clang.
* m4/isnanf.m4 (gl_HAVE_ISNANF_NO_LIBM, gl_ISNANF_WORKS): Use the GCC
built-in __builtin_isnan, not __builtin_isnanf. Also on clang.
* lib/isnand-nolibm.h (isnand): With clang, use the GCC built-in.
* m4/isnand.m4 (gl_HAVE_ISNAND_IN_LIBM): With clang, use the GCC
built-in.
* lib/isnanl-nolibm.h (isnanf): Use the GCC built-in __builtin_isnan,
not __builtin_isnanl. Also on clang.
* m4/isnanl.m4 (gl_HAVE_ISNANL_NO_LIBM, gl_FUNC_ISNANL_WORKS): Use the
GCC built-in __builtin_isnan, not __builtin_isnanl. Also on clang.
* lib/math.in.h (__has_builtin): Remove macro.
(isnanf, gl_isnan_f): Use the GCC built-in __builtin_isnan, not
__builtin_isnanf. Also on clang.
(isnand, gl_isnan_d): With clang, use the GCC built-in.
(isnanl, gl_isnan_l): Use the GCC built-in __builtin_isnan, not
__builtin_isnanl. Also on clang.
(isnan): Use the GCC built-in __builtin_isnan in all three cases. Also
on clang.