mingw: Define PGDLLEXPORT as __declspec (dllexport) as done for msvc

mingw: Define PGDLLEXPORT as __declspec (dllexport) as done for msvc

While mingw would otherwise fall back to
__attribute__((visibility("default"))), that appears to only work as long as
no symbols are declared with __declspec(dllexport). But we can end up with
some, e.g. plpython's Py_Init.

It's quite possible we should do the same for cygwin, but I don't have a test
environment for that...

Discussion: http://postgr.es/m/20220928022724.erzuk5v4ai4b53do@awork3.anarazel.de
Discussion: http://postgr.es/m/20220928025242.ugf7t5ugxxgmkraa@awork3.anarazel.de