When the GNAT driver is called with a project file and a single main
specified as an absolute path, the specific switches that are declared
for the main source were not taken into account. This patch fixes this.
The specific switches are now taken into account.

Example:

prj.gpr:
project Prj is
   package Pretty_Printer is
      for Default_Switches ("Ada") use ("-nD");
      for Switches ("pkg.ads") use ("-kU");
   end Pretty_Printer;
end Prj;

Invoking "gnat pretty -P prj.gpr /path/to/pkg.ads" should result in
gnatpp invoked with -aU, not -nD.

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

2013-01-03  Vincent Celier  <celier@adacore.com>

	* gnatcmd.adb (GNATCmd): If a single main has been specified
	as an absolute path, use its simple file name to find specific
	switches, instead of the absolute path.