Initial commit
Teach BundleUtilities and GetPrerequisites how to fixup bundle using @rpath.

The following functions and variables have been introduced:

  IS_RESOLVED_ITEM_EMBEDDED (BundleUtilities)
  BU_RESOLVE_ITEM_RPATH_DIRS_OVERRIDE (BundleUtilities)
  GP_RESOLVE_ITEM_RPATH_DIRS (GetPrerequisites)

In function "set_bundle_key_values":

 1) automatically compute the relative
path of executable to the bundle directory. Doing so allow to remove
the hack introduced in commit 6f8bdd27 consisting in
reseting the value of 'resolved_embedded_item' with 'resolved_item'.

 2) Also use the function "gp_resolve_item" to
resolve the emdedded_item. Before this commit, the function was able to
resolve only item matching "@executable_path". In order to be able
to successfully use the "gp_resolve_item" for item matching "@rpath", the
variable "GP_RESOLVE_ITEM_RPATH_DIRS" has been introduced.
By setting this variable, gp_resolve_item can resolve the rpath item
successfully. The list of path to use usually correspond to the directories
 passed to the 'rpath' linker option.

Note also that the BundleUtilities script is doing its best to compute
the value associated to GP_RESOLVE_ITEM_RPATH_DIRS. That said, the
user can still override the automatically computed value by setting
BU_RESOLVE_ITEM_RPATH_DIRS_OVERRIDE
BundleUtilities - Factor out code checking if resolved item it embedded
BundleUtilities - Do not copy item if they are already embedded