Zexy update to 2.3.1
Update zexy to the latest IEM version, which also works in the mingw64 build. (Our current version is old, and while it compiles with mingw64, none of the objects will load.)
Some other advantages of the new version are:
Zexy is in its own external library now, so its dozens of objects load much quicker.
The baroque automake build system was replaced with a pdlibbuilder Makefile, so the zexy target is much simpler now and compiles much quicker, especially on Windows.
Note that I simply assumed that the bugs in our previous version should have been fixed by now, so I didn't check whether we may have to reapply some of the changes that went into the old version beginning with rev. 7b3a9f53. If anyone wants to do that, please be my guest. ;-)