_try_read: hacky fix to make GNU make less angry.

We'll have to stop using nonblocking reads, unfortunately.  But this seems
to work better than nothing.  There's still a race condition that could
theoretically make GNU make angry, unfortunately, since we briefly set the
socket to nonblocking.

jwack.py

@@ -32,14 +32,20 @@ def _release(n):
 
 
 def _try_read(fd, n):
+    # FIXME: this isn't actually safe, because GNU make can't handle it if
+    # the socket is nonblocking.  Ugh.  That means we'll have to do their
+    # horrible SIGCHLD hack after all.
     fcntl.fcntl(_fds[0], fcntl.F_SETFL, os.O_NONBLOCK)
     try:
-        b = os.read(_fds[0], 1)  # FIXME try: block
+        try:
+            b = os.read(_fds[0], 1)
         except OSError, e:
             if e.errno == errno.EAGAIN:
                 return ''
             else:
                 raise
+    finally:
+        fcntl.fcntl(_fds[0], fcntl.F_SETFL, 0)
     return b and b or None