Hook error() and error_at_line() if available.

They are used by some GNU programs to display error messages.

tests/Makefile.am

@@ -4,8 +4,14 @@ example_SOURCES = example.c
 example_exec_SOURCES = example_exec.c
 example_vfork_SOURCES = example_vfork.c
 
+if HAVE_ERROR_H
+    check_PROGRAMS += example_error
+    example_error_SOURCES = example_error.c
+endif
+
 dist_check_SCRIPTS = run.sh lib.sh
 dist_check_DATA = example.expected \
+                  example_error.expected \
                   example_exec.expected \
                   example_vfork.expected \
                   example-noforce.sh \