* lib/math.in.h (signbit): Use __builtin_signbit{,f,l} also on clang.
* m4/signbit.m4 (gl_SIGNBIT): Use __builtin_signbit{,f,l} also on clang.
Set REPLACE_SIGNBIT_USING_BUILTINS, not REPLACE_SIGNBIT_USING_GCC.
* m4/math_h.m4 (gl_MATH_H_DEFAULTS): Initialize
REPLACE_SIGNBIT_USING_BUILTINS, not REPLACE_SIGNBIT_USING_GCC.
* modules/math (Makefile.am): Substitute REPLACE_SIGNBIT_USING_BUILTINS,
not REPLACE_SIGNBIT_USING_GCC.