This patch corrects an issue whereby source files that did not have read
permissions were incorrectly referred to as "not found'. Now, these different
cases are explicitly identified and warned about properly.

------------
-- Source --
------------

--  toto.c

void toto(void)
{
}

--  hello.adb

with Ada.Text_IO; use Ada.Text_IO;
procedure Hello is
begin
   Put_Line ("Hello, world!");
end Hello;

----------------------------
-- Compilation and output --
----------------------------

& chmod a-r hello.adb
& gcc -c hello.adb
& chmod a+r hello.adb
& chmod a-r toto.c
& gcc -c toto.c
& chmod a+r toto.c

no read access for file "hello.adb"
cc1: fatal error: toto.c: Permission denied
compilation terminated.

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

2017-09-29  Justin Squirek  <squirek@adacore.com>

	* ali-util.adb, comperr.adb, errout.adb, fmap.adb, fname-sf.adb,
	frontend.adb, lib-xref-spark_specific.adb, gnat1drv.adb, gnatls.adb,
	lib.adb, lib-load.adb, lib-writ.adb, prepcomp.adb, sinput-d.adb,
	sinput-l.adb, sprint.adb, targparm.adb: Update comparison for checking
	source file status and error message and/or call to Read_Source_File.
	* libgnat/s-os_lib.ads: Add new potential value constant for
	uninitialized file descriptors.
	* osint.adb, osint.ads (Read_Source_File): Add extra parameter to
	return result of IO to encompass a read access failure in addition to a
	file-not-found error.