meson: mingw: Allow multiple definitions
I didn't carry this forward from the win32 template. It's not needed anymore
for the reason stated therein, but it turns out to be required to
e.g. override getopt. Possibly a better solution exists, but that's for later.
Discussion: http://postgr.es/m/
20220928022724.erzuk5v4ai4b53do@awork3.anarazel.de