configure: split has_or_abort in two: one to check, one to abort

Split the has_or_abort function in two:
- one that checks if the tool if found,
- one that calls the above check, and aborts if not found

The rational behind this is to be able to check for a tool
and if not found, fallback to using our bundled version,
should the need arise (and I get time).

hg