Fix a build failure on mingw with -O0.
Reported at <https://savannah.gnu.org/bugs/?36443>.
Fix suggested by Evgeny Grin <k2k@narod.ru>
in <https://savannah.gnu.org/bugs/?36443#comment4>.
* gnulib-local/build-aux/moopp (func_emit_source_h): Wrap the function
declarations in extern "C" {}.
