Add CMake option to enable searching for DynLibs in an extra folder

[hhyyrylainen]

This adds an CMake option that when specifies enables the DynLib
loader to look for the library in an additional folder.

I was unable to fully test this as it seems that even though I have my
libs in a sub folder, it seems that just loading them by name
works. Maybe my RPATH setting comes into play here? This is at least
so on Linux.

[BearishSun]

Hi. Can you provide a use-case for this functionality?

[hhyyrylainen]

Not having to clutter the same folder as the executable with a bunch of libraries is nice. I coded this before I found out that at least on my computer the ld open lib function seems to take RPATH into account.

[BearishSun]

Makes sense. Although I'd prefer this path is something we provide to DynLib from higher level systems when plugins are being loaded, rather than adding it under the hood. Not sure if it's required to be a build option? I'd prefer we add a path to START_UP_DESC which can then used in DynLib calls for plugins, as required.

[hhyyrylainen]

I suppose it makes more sense to have this in the START_UP_DESC. I was in the mindset of adding cmake flags, as I'm going to next do a PR for adding flags for enabling RTTI and exceptions, and leaving debug info alone. So I just made this a flag as well.