Packaging tweaks

A few tweaks for packaging:

• the otbenv scripts now check if variables PATH, PYTHONPATH and OTB_APPLICATION_PATH are not empty. The PYTHONPATH with a trailing : was causing problems with git-bash.
• in the standalone macros, there was a missing include to GenerateExportHeaderCustom
• in the standalone Python wrappers building scripts, we now set the RPATH during install (it is usually done by the .run script)