Properly generate compat.h

[withoutboats]

My build script was so simple! Alas!

liburing used to contain a src/include/liburing/compat.h header file. However, it now generates that compat.h header file using a script called configure. And cargo does not want users to write any files outside of $OUT_DIR.

I think I need to make it so it can call configure to write into outdir, which unfortunately I think means copying configure into OUT_DIR and then running it there, and then I need to make outdir an include directory for my build.

This is all becoming quite a headache honestly.