Two new directories are added in the project path, when gnatls is invoked
with --RTS=, just before the two directories for the target.
When the runtime is a single name, the directories are:
  <prefix>/<target>/<runtime>/lib/gnat
  <prefix>/<target>/<runtime>/share/gpr
Otherwise, the runtime directory is either an absolute path or a path
relative to the current working directory and the two added directories
are:
  <runtime_directory>/lib/gnat
  <runtime_directory>/share/gpr

Tested on x86_64-pc-linux-gnu, committed on trunk

2015-01-06  Vincent Celier  <celier@adacore.com>

	* gnatls.adb (Search_RTS): Invoke Initialize_Default_Project_Path
	with the runtime name.
	* prj-env.adb (Initialize_Default_Project_Path): When both
	Target_Name and Runtime_Name are not empty string, add to the
	project path the two directories .../lib/gnat and .../share/gpr
	related to the runtime.
	* prj-env.ads (Initialize_Default_Project_Path): New String
	parameter Runtime_Name, defaulted to the empty string.