Delegates libusb external communication to externals/CMakeLists.txt
Ensures an interface library `usb` for every pathway
input_common just links to the `usb` library now
externals/libusb/CMakeLists.txt sets variables to override SDL2's libusb
finding
Other minor cleanup
Building libusb was also broken on GCC (and maybe Clang) on our
CMakeLists after upgrading to 1.0.24, but it was not being checked
because our 18.04 container had libusb installed on it.
This builds on the MinGW work from earlier and extends it to the rest of
the GNU toolchains. In addition we make use of pkg-config when present
to find libusb. pkg-config is preferrable because we can specify a
minimum required version.
After updating to 1.0.24, MinGW fails to build libusb as a result of
numerous errors. So we build libusb their way and let them update the
nontrivial stuff.
This only applies to MinGW: the old path is still in use for Linux
toolchains as well as MSVC.
This will dynamically link libusb, since I hit build errors with the old
way we used to resolve the conflict with SDL2.
Given we have two libraries that seem to use the same identifier, we can
alter one of them so that the variable is used in place, effectively
changing the used identifier, but without altering the source of
libusb.
We can place the external in an inner folder and manage the custom files
necessary to integrate it with CMake directly. This allows us to
directly change how we use it with our build system, as opposed to
needing to change a fork.
