variety of platforms. A few are missing, and they will be added in
eventually, but as this is new stuff, it was better to not break lots of
platforms in one go that we can't easily test. The changes to "Configure"
should illustrate how to add support to other systems if you feel like
having a go.

NB: I'll add something shortly to allow you to add "dlfcn.h" support on
those platforms that don't have (or need) a dlfcn.h header file. (The
symbol for Configure will probably by "dlfcn_no_h").

Thanks to Richard Levitte, who is responsible for the dso_dl.c support,
understanding the trickier aspects of the build process, and giving great
feedback on everything else.

[Don't use this stuff if you're easily offended by changes to the
interface or behaviour - it's still work in progress.]

PR: