[Ada] New environmment variable GPR_PROJECT_PATH_FILE for project path

[Arnaud Charlet <charlet at adacore dot com>]

A new environment variable GPR_PROJECT_PATH_FILE is used by the Project
Manager to set up the project path. When defined, GPR_PROJECT_PATH_FILE
indicates the path of a text file that contains directories to be added
to the project path. GPR_PROJECT_PATH_FILE is taken into account before
GPR_PROJECT_PATH.
The test for this is to create a text file containing the path names
of project directories, to define GPR_PROJECT_PATH_FILE with the path
name of this file and to invoke gnatmake with project files that need
to have the project path correctly defined.

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

2013-04-11  Vincent Celier  <celier@adacore.com>

	* prj-env.adb (Initialize_Default_Project_Path): Take
	into account a project path file, specified by environment
	variable GPR_PROJECT_PATH_FILE, before taking into account
	GPR_PROJECT_PATH.
	* projects.texi: Add documentation for GPR_PROJECT_PATH_FILE

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