Search for headers relative to project root

The find module may be called from a CMakeLists.txt in a subdirectory,
and it would start the search from there.
Allow lowercase variables for specifying project root
Don't search relative directories if root is given

The PATHS that are specified is always searched, also if NO_DEFAULT_PATH
is given.
Allow package to be specified as required

Although CMake prefers the uppercase variant variables, and the
find_package_handle_standard_args will convert to them for us, the
part that parses REQUIRED and QUIET arguments to find_package still
only uses the package name verbatim (sic).
Test for all the required header directories
Merge pull request #38 from rolk/038_cmake

Allow directory root to be specified as the rest of the OPM modules