[Ada] Check for attempt to bind GNATprove files

[Arnaud Charlet <charlet at adacore dot com>]

If one or more objects is compiled in GNATprove mode (either by using
GNATprove directly, or by using -gnatd.F), then the ALI file is marked
and gnatbind will exit with a message as shown here. Given:

     1. procedure linkdf is
     2. begin
     3.    null;
     4. end;

If we first compile this with

  gcc -c linkdf.adb -gnatd.F

then we try to do a gnatmake, we get

error: one or more files compiled in GNATprove mode
gnatmake: *** bind failed.

Previously this was not detected and the linker bombed
with peculiar error messages.

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

2014-10-10  Robert Dewar  <dewar@adacore.com>

	* ali.adb (Scan_ALI): Read and process new GP flag on ALI P line.
	* ali.ads (GNATprove_Mode): New component in ALI table.
	(GNATprove_Mode_Specified): New global.
	* gnatbind.adb (Gnatbind): Give fatal error if any file compiled
	in GNATProve mode.
	* lib-writ.ads, lib-writ.adb (GP): New flag on P line for
	GNATProve_Mode.

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