opt.ads opt.adb: Add Ada_Version_Explicit_Config along with save/restore routines.

opt.ads opt.adb: Add Ada_Version_Explicit_Config along with save/restore routines.

2006-02-13  Robert Dewar  <dewar@adacore.com>
	    Serguei Rybin  <rybin@adacore.com>

	* opt.ads opt.adb: Add Ada_Version_Explicit_Config along with
	save/restore routines.
	Properly handle Ada_Version_Explicit and Ada_Version_Config, which
	were not always properly handled previously.
	Since we are changing the tree format anyway, also get rid of the
	junk obsolete Immediate_Errors flag.
	(Tree_Read): Change the way of reading Tree_Version_String - now we
	read the version string from the tree even if its length is not the
	same as the length of the version string computed from Gnatvsn.
	(Search_Directory_Present): New Boolean flag for the compiler.
	Define Tree_Version_String as a dynamic string.
	(Default_Stack_Size): new variable, used to handle switch -d.

	* par-prag.adb:
	For pragma Ada_2005, remove stuff about setting Ada_Version_Explicit
	only for main unit.
	Add pragma Ada_2005 (synonym for Ada_05)
	Properly handle Ada_Version_Explicit and Ada_Version_Config, which
	were not always properly handled previously.

	* directio.ads, ioexcept.ads, sequenio.ads, text_io.ads: Change
	explicit Ada_95 to Ada_2005.

From-SVN: r111054