Make it possible to install the generated headers

Adding a header_path parameter. This will cause the scanner to generate
header includes that look for corresponding files in that path.

Also when we'r installing files, then c++ classes needs to be exported

Change-Id: Ic539c0861143d7200b9cae85ca902a17ea0d0a8d
Reviewed-by: Andy Nichols <andy.nichols@digia.com>