major rationalization of use of search paths. ardour now has just 4 functions used...

major rationalization of use of search paths. ardour now has just 4 functions used to define how external resources are located: ardour_config_search_path() (for system or user specific configuration data), ardour_data_search_path() (for machine, user and system independent data), ardour_dll_directory() (base directory where shared libraries are found) and user_config_directory(). These are now used throughout the code. the config, data and dll paths/directories can be overridden by environment variables. the user config dir is added as the first element of the first two search paths, and use selectively when searching for a few other things.
This commit re-enabes ./waf install, and it is believed that it works fully at this point (more testing likely required)

git-svn-id: svn://localhost/ardour2/branches/3.0@12326 d708f5d6-7413-0410-9779-e7cbd77b26cf