Fix Ada bootstrap issue

[Eric Botcazou]

Because of the recent reorganization of the ada/ directory, the check for the 
presence of a working Ada compiler fails in stage #2 and later if you don't 
have a compiler already installed in the --prefix directory.

Bootstrapped on x86_64-suse-linux, applied on the mainline.


2017-10-20  Nicolas Roche  <ro...@adacore.com>

        * configure.ac (ACX_PROG_GNAT): Append "libgnat" to include search dir
        * configure: Regenerate.

-- 
Eric Botcazou
Index: configure.ac
===================================================================
--- configure.ac	(revision 253921)
+++ configure.ac	(working copy)
@@ -362,7 +362,7 @@ rm -f a.out a.exe b.out
 # Find the native compiler
 AC_PROG_CC
 AC_PROG_CXX
-ACX_PROG_GNAT([-I"$srcdir"/ada])
+ACX_PROG_GNAT([-I"$srcdir"/ada/libgnat])
 
 # Do configure tests with the C++ compiler, since that's what we build with.
 AC_LANG(C++)