Don't check for stpcpy.

* configure.ac: Don't check for stpcpy.
* src/rfc822parse.c (my_stpcpy): Define our own stpcpy
  unconditionally.

Windows does not have stpcpy and It's not part of mingw. The
function check failed accordingly but GCC still warned about
shadowing a builtin function.

Just avoid this trouble and always define our own function.