Determine that ERT is not found, faster

Instead of checking every pre-requisite and then determine if the
library is functional, quickly check if the library directories are
present, and if not bail out. The most common reason that ERT is
not usuable, is that it is installed, not that something else is
missing!
Merge pull request #177 from rolk/177_fastert

Determine that ERT is not found, faster