totalorderl: Simplify on MSVC.

totalorderl: Simplify on MSVC.

* lib/totalorderl.c (totalorderl): If 'long double' is the same as
'double', just invoke totalorder. Otherwise, drop the optimized code
for small 'long double' since it does not occur on any platform.
* m4/totalorder.m4 (gl_FUNC_TOTALORDERL): Require
gl_LONG_DOUBLE_VS_DOUBLE. If 'long double' is the same as 'double', use
TOTALORDER_LIBM.
* modules/totalorderl (Depends-on): Add totalorder.