OMakeroot: fix install bug
OMakeroot: fix install bug

OMake doesn’t create symlinks to all source files if they aren’t
needed. For example, future_async.mli doesn’t exist under _build if
COMPILE_LIB_ASYNC=false. This would cause `omake install` to error out.
Could put mli files under -conditional option, but this works too.
Closes #7.

struktured/future