Use automake silent rules if available

If available, default to use silent automake rules.
This makes the result of `make` less verbose and more
concise, allowing for warnings to be much easier to spot.
The previous behaviour can still be anabled by running
`make V=1`.

Example:

$ make
make  all-am
make[1]: Entering directory `/home/fdmanana/git/hub/membase/couchstore'
  CC     src/libcouchstore_la-arena.lo
  CC     src/libcouchstore_la-btree_modify.lo
  CC     src/libcouchstore_la-btree_read.lo
  CC     src/libcouchstore_la-couch_db.lo
  CC     src/libcouchstore_la-couch_save.lo
  CC     src/libcouchstore_la-crc32.lo
  CC     src/libcouchstore_la-couch_file_read.lo
  CC     src/libcouchstore_la-couch_file_write.lo
  CC     src/libcouchstore_la-db_compact.lo
  CC     src/libcouchstore_la-iobuffer.lo
  CC     src/libcouchstore_la-json_reduce.lo
src/json_reduce.c: In function 'add_count':
src/json_reduce.c:52:57: warning: unused parameter 'key' [-Wunused-parameter]
src/json_reduce.c:52:78: warning: unused parameter 'value' [-Wunused-parameter]
src/json_reduce.c: In function 'finish_count':
src/json_reduce.c:68:5: warning: format '%llu' expects argument of type 'long long unsigned int', but argument 3 has type 'uint64_t' [-Wformat]
src/json_reduce.c: In function 'add_sum':
src/json_reduce.c:82:55: warning: unused parameter 'key' [-Wunused-parameter]
src/json_reduce.c: In function 'add_stats':
src/json_reduce.c:119:57: warning: unused parameter 'key' [-Wunused-parameter]
src/json_reduce.c: In function 'add_reduced_stats':
src/json_reduce.c:141:26: warning: format '%llu' expects argument of type 'long long unsigned int *', but argument 3 has type 'uint64_t *' [-Wformat]
src/json_reduce.c: In function 'finish_stats':
src/json_reduce.c:159:20: warning: format '%llu' expects argument of type 'long long unsigned int', but argument 3 has type 'uint64_t' [-Wformat]
  CC     src/libcouchstore_la-llmsort.lo
  CC     src/libcouchstore_la-tree_writer.lo
  CC     src/libcouchstore_la-mergesort.lo
  CC     src/libcouchstore_la-node_types.lo
  CC     src/libcouchstore_la-reduces.lo
  CC     src/libcouchstore_la-strerror.lo
  CC     src/libcouchstore_la-util.lo
  CC     src/libcouchstore_la-couch_index.lo
src/couch_index.c: In function 'primary_reduce_common':
src/couch_index.c:267:5: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
src/couch_index.c:311:13: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
  CC     src/views/libcouchstore_la-collate_json.lo
  CC     src/views/libcouchstore_la-bitmap.lo
  CC     src/views/libcouchstore_la-index_header.lo
  CC     src/views/libcouchstore_la-sorted_list.lo
  CC     src/views/libcouchstore_la-reductions.lo
  CC     src/views/libcouchstore_la-keys.lo
  CC     src/views/libcouchstore_la-values.lo
  CC     src/libcouchstore_la-os.lo
src/os.c: In function 'couch_close':
src/os.c:96:7: warning: 'rv' may be used uninitialized in this function [-Wuninitialized]
  CC     src/rfc1321/librfc1321_la-md5c.lo
  CCLD   librfc1321.la
  CC     src/byteswap.lo
  CCLD   libbyteswap.la
  CXX    src/views/mapreduce/libmapreduce_la-mapreduce_c.lo
  CXX    src/views/mapreduce/libmapreduce_la-mapreduce.lo
  CXXLD  libmapreduce.la
  CCLD   libcouchstore.la
  CC     src/couch_dbdump-dbdump.o
src/dbdump.c: In function ‘visit_node’:
src/dbdump.c:133:9: warning: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 2 has type ‘uint64_t’ [-Wformat]
src/dbdump.c:137:9: warning: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 3 has type ‘size_t’ [-Wformat]
src/dbdump.c:139:13: warning: format ‘%lld’ expects argument of type ‘long long int’, but argument 2 has type ‘uint64_t’ [-Wformat]
src/dbdump.c: In function ‘main’:
src/dbdump.c:210:14: warning: variable ‘mode’ set but not used [-Wunused-but-set-variable]
  CCLD   couch_dbdump
  CC     src/couch_dbinfo-dbinfo.o
  CC     src/couch_dbinfo-util.o
  CCLD   couch_dbinfo
  CC     src/couch_compact-compactor.o
  CCLD   couch_compact
  CC     src/couch_viewgen-viewgen.o
  CCLD   couch_viewgen
  CXX    src/couchscript-couchscript.o
src/couchscript.cc: In function ‘int couch_get(lua_State*)’:
src/couchscript.cc:206:88: warning: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 5 has type ‘uint64_t {aka long unsigned int}’ [-Wformat]
src/couchscript.cc:206:88: warning: format ‘%lld’ expects argument of type ‘long long int’, but argument 6 has type ‘size_t {aka long unsigned int}’ [-Wformat]
src/couchscript.cc:206:88: warning: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 5 has type ‘uint64_t {aka long unsigned int}’ [-Wformat]
src/couchscript.cc:206:88: warning: format ‘%lld’ expects argument of type ‘long long int’, but argument 6 has type ‘size_t {aka long unsigned int}’ [-Wformat]
src/couchscript.cc: In function ‘int couch_truncate(lua_State*)’:
src/couchscript.cc:246:60: warning: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 5 has type ‘cs_off_t {aka long int}’ [-Wformat]
src/couchscript.cc:246:60: warning: format ‘%llu’ expects argument of type ‘long long unsigned int’, but argument 5 has type ‘cs_off_t {aka long int}’ [-Wformat]
  CXXLD  couchscript
make[1]: Leaving directory `/home/fdmanana/git/hub/membase/couchstore'

Change-Id: If64b76da4f11fd1e4c4ea42ba0f07cf1ed894cee
Reviewed-on: http://review.couchbase.org/25633
Reviewed-by: Fulu Li <fulu@couchbase.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>