build: Don't rename error{_at_line} to rpl_error{_at_line} on native Windows.
* gettext-tools/configure.ac (GNULIB_REPLACE_ERROR): Define only on Cygwin and
Android.
* gettext-tools/woe32dll/gettextlib-exports.c: Don't assume that
GNULIB_DEFINED_ERROR implies GNULIB_REPLACE_ERROR.
