mingw: Dynamically link to _mkgmtime().
mingw: Dynamically link to _mkgmtime().

Cygwin's Mingw-w64 cross compiler tools are unable to link against
_mkgmtime() (though the function is declared at compile time). To work
around this, the linking is done dynamically at runtime (based on code
at "Using Run-Time Dynamic Linking",
https://msdn.microsoft.com/en-us/library/windows/desktop/ms686944(v=vs.85).aspx).

As there is no way to differentiate between build tools that can
statically link to the function and those who can't, dynamic linking is
done for all MinGW targets. Those who wish to statically link to the
function, and can do so, can set 'HAVE_MKGMTIME' at compile time.

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