X11Utils: Don't depend on wx

We want to have two build targets: Main and MainNoGUI, and this code
will be linked against in both cases, so the ifdef isn't enough for
this case.

Just append to a vector of strings, and then convert it after the fact.
DolphinWX: Allow building both GUI and NoGUI at the same time

Restructure our build system so we have multiple targets. Right now we
only build MainNoGUI if we are using X11, since that's the only truly
supported backend: the OS X code actually doesn't compile, according to
comments made on IRC.
CMakeLists: Try to fix the X11 detection on OS X