Add simple config for symlink creation

[Jake-Shadle]

Right now one needs to manually change the code to address issues where the casing of a library or header used by external code doesn't match the file or symlink(s) created by xwin, which is...tedious. I still think using symlinks to transparently fixup these issues is the best approach, rather than requiring users to install the CRT/SDK within a case-insensitive filesystem/directory, but this issue will keep coming up since there is nothing stopping natively compiled Windows code from linking/including all kinds of crazily cased filenames, so it makes sense to make this more easily configurable. It would also be a way for users to supply their own configuration file at runtime to immediately fix issues until their fixup can be included in the main xwin release so that others get it for free.