(maint) Fix build dependency finding to support CMake hint variables.

Adding back support for a <packagename>_ROOT CMake variable that
gives a hint to where dependencies can be located.

Also adding support for <packagename>_INCLUDEDIR for an include
directory hint and a <packagename>_LIBRARYDIR for a library
directory hint.
(maint) Add missing doc comment for time zone fact resolver.

Adding a missing doc comment for timezone_resolver.
Merge pull request #102 from peterhuene/maint/dependencies

(maint) Fix build dependency finding to support CMake hint variables.