CONFIG: update infiniband configuration macro

* What: infiniband is configured to be used just by looking the
existence of the ibverbs stack even if there are no devices
* When: systems with installed ibverbs and no `--with-ethernet`
explicitly setup
* By: create a explicit test for actual infiniband devices, printing a
warning if there are not
* But: the original is left untouched, thus cross-compiling is still
allowed