Enable automake's "silent rules" if they are available.

Automake version 1.11 and above provide a build mode where
each build command is replaced by a single-line annotation.
Since autotools build lines for libraries typically each fill
an entire terminal screen, this mode makes it easier to keep
track of build progress, and warnings stand out more clearly.

For debugging the build process, the old behaviour can be
recovered by running 'make V=1' at bulid time, or by passing
--disable-silent-rules to configure.

Since Automake 1.11 is still very recent, we only invoke
the AM_SILENT_RULES macro if it is defined, allowing fallback
to the old behaviour under previous versions of automake.

configure.ac

@@ -4,6 +4,9 @@ AC_INIT(src/opus_encoder.c)
 
 AM_CONFIG_HEADER([config.h])
 
+dnl enable silent rules on automake 1.11 and later
+m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
+
 OPUS_MAJOR_VERSION=0
 OPUS_MINOR_VERSION=9
 OPUS_MICRO_VERSION=5