Add "../lib/unbuffer_output.c" and use it in gdb.base/interrupt.c

Add "../lib/unbuffer_output.c" and use it in gdb.base/interrupt.c

In some scenarios, GDB or GDBserver can be spawned with input _not_
connected to a tty, and then tests that rely on stdio fail with
timeouts, because the inferior's stdout and stderr streams end up
fully buffered.

See discussion here:
  https://sourceware.org/ml/gdb-patches/2015-02/msg00809.html

We have a hack in place that works around this for Windows testing,
that forces every test program to link with an .o file that does
(lib/set_unbuffered_mode.c):

 static int __gdb_set_unbuffered_output (void) __attribute__ ((constructor));
 static int
 __gdb_set_unbuffered_output (void)
 {
   setvbuf (stdout, NULL, _IONBF, BUFSIZ);
   setvbuf (stderr, NULL, _IONBF, BUFSIZ);
 }

That's a bit hacky; it ends up done for _all_ tests.

This patch adds a way to do this unbuffering explicitly from the test
code itself, so it is done only when necessary, and for all
targets/hosts.  For starters, it adjusts gdb.base/interrupt.c to use
it.

Tested on x86_64 Fedora 20, native, and against a remote gdbserver
board file that connects to the target with ssh, with and without -t
(create pty).

gdb/testsuite/
2015-02-27  Pedro Alves  <palves@redhat.com>

	* lib/unbuffer_output.c: New file.
	* gdb.base/interrupt.c: Include "../lib/unbuffer_output.c".
	(main): Call gdb_unbuffer_output.