Explicitly specified paths should replace predefined ones

We don't want a typo in any of those to accidentially make the
configuration pick up the system libraries instead.
Look for packages from underneath a single root

Previous implementation had a separate search path for headers and for
the library, which in the case of "RedHat-multiarch" made it miss the
library (it was looking for /usr/lib/lib64 instead of /usr/lib64); the
new version starts looking from /usr.
Add realtime library on Linux, if SuiteSparse >= 4.0

Functions in the helper library is dependent on librt