Shlib exports file refactoring

Peter Eisentraut <peter_e@gmx.net> writes:

After seeing four nearly-identical copies of multiplatform shared library
exports file generation code, I figured this should be put in a common place.
If you like, please test the attached patch on darwin and win32, since these
platforms are mostly affected (besides linux) and I can't test them. Just
see if it builds and runs correctly. Thanks.

[ shrug... ] Apply it to HEAD and see if the buildfarm complains.
We're in devel cycle now, so transient breakage isn't going to be a big
problem. If you were asking for tests that the buildfarm wouldn't make,
I might think differently ... but then you'd need to be giving some more
detailed directions than these.

regards, tom lane