x11 for linux screenshot, keyscan_dump and clipboard

[timwr]

We should be able to use x11 (as a non-root user) to implement the screenshot, keyscan and clipboard APIs on mettle. I made an attempt here:
https://github.com/timwr/mettle/tree/x11
I started to link libx11 but I thought I'd check with you guys whether this is the correct way forward. Perhaps it's better just to include the headers and load libX11 with dlopen/dlsym (assuming dlopen is even implemented)?