meson: Don't use strndup() unless it's prototyped

A workaround for https://github.com/mesonbuild/meson/issues/3672

MinGW-w64 gcc has a built-in strndup, but it's not in the C library and
MinGW-w64 headers don't prototype it.

Don't try to use it, as that will cause an undefined reference if gcc
decides that an out-of-line call is appropriate.
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