gccrs: Add config-lang.in

This was a copy/paste from gccgo front-end. We do not use any of the
target_libs yet, but we will need these when we support the libpanic crate.

	gcc/rust/
	* config-lang.in: New.

gcc/rust/config-lang.in

@@ -0,0 +1,34 @@
+# config-lang.in -- Top level configure fragment for gcc Rust frontend.
+
+# Copyright (C) 2009-2022 Free Software Foundation, Inc.
+
+# This file is part of GCC.
+
+# GCC is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3, or (at your option)
+# any later version.
+
+# GCC is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with GCC; see the file COPYING3.  If not see
+# <http://www.gnu.org/licenses/>.
+
+# Configure looks for the existence of this file to auto-config each language.
+# We define several parameters used by configure:
+#
+# language	- name of language as it would appear in $(LANGUAGES)
+# compilers	- value to add to $(COMPILERS)
+
+language="rust"
+compilers="rust1\$(exeext)"
+
+build_by_default="no"
+
+target_libs="target-libffi target-libbacktrace"
+
+gtfiles="\$(srcdir)/rust/rust-lang.cc"