Use __builtin_stpcpy only if the system supports stpcpy.
This is necessary as in some cases a call to stpcpy will
be emitted if the built-in can not optimized.

include/m_string.h:
  The expansion of stpcpy (in glibc) causes warnings if the
  return value of strmov is not being used. Since stpcpy is
  a GNU extension and the expansion ends up using a built-in
  provided by GCC, use the compiler provided built-in directly
  when possible. Nonetheless, the C library must have stpcpy
  as a call be emitted if the built-in can not optimized.