isnanf, isnanl, isnan: Don't use nonexistent builtins with clang.
* m4/isnanf.m4 (gl_HAVE_ISNANF_NO_LIBM, gl_HAVE_ISNANF_IN_LIBM,
gl_ISNANF_WORKS): Don't use __builtin_isnanf on clang versions that
don't have it.
* m4/isnanl.m4 (gl_HAVE_ISNANL_NO_LIBM, gl_HAVE_ISNANL_IN_LIBM,
gl_FUNC_ISNANL_WORKS): Don't use __builtin_isnanl on clang versions that
don't have it.
* lib/isnanf-nolibm.h (__has_builtin): New macro.
(isnanf): Don't use __builtin_isnanf on clang versions that don't have
it.
* lib/isnanl-nolibm.h (__has_builtin): New macro.
(isnanl): Don't use __builtin_isnanl on clang versions that don't have
it.
* lib/math.in.h (__has_builtin): New macro.
(isnanf): Don't use __builtin_isnanf on clang versions that don't have
it.
(isnanl): Don't use __builtin_isnanl on clang versions that don't have
it.
(isnan): Don't use the builtins on clang versions that don't have
__builtin_isnanf and __builtin_isnanl.