build: enable AM_SILENT_RULES

This shuts up make by default (can be reversed with "make V=1" or
--disable-silent-rules).  This is useful since warnings and error
messages become more visible with less noise.

Tested on Linux with GNU make and FreeBSD with system's BSD make.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
Acked-by: Paul Jakma <paul@jakma.org>
diff --git a/configure.ac b/configure.ac
index 20d21f6..eeb2420 100755
--- a/configure.ac
+++ b/configure.ac
@@ -21,6 +21,7 @@
 AC_CANONICAL_TARGET()
 
 AM_INIT_AUTOMAKE(1.6)
+AM_SILENT_RULES([yes])
 AC_CONFIG_HEADERS(config.h)
 
 AC_PATH_PROG(PERL, perl)