Enable the automake silent rules.

$ make V=1 # will temporarily disable them

otherwise you see:

   CC foo.c

rather than the giant long bulid line.

This makes it significantly easier to spot compiler warnings etc.

Additionally, make them conditional, so we won't error on automake <
1.11

(commits squashed by nickm.)

changes/tkt6522

+  o Build changes:
+    - Where available, we now use automake's "silent" make rules by
+      default, so that warnings are easier to spot.  You can get the
+      old behavior with "make V=1". Patch by Stewart Smith for ticket
+      6522.

configure.in

@@ -5,6 +5,7 @@ dnl See LICENSE for licensing information
 
 AC_INIT
 AM_INIT_AUTOMAKE(tor, 0.2.4.0-alpha-dev)
+m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 AM_CONFIG_HEADER(orconfig.h)
 
 AC_CANONICAL_HOST