configure.ac: use silent rules if available

Using silent rules gives a cleaner output from the build.  This makes it
trivially easy to spot warnings.  The original, verbose, output can still
be seen by doing one of these...

$ ./configure --disable-silent-rules && make

...or...

$ ./configure && make V=1

...otherwise the build output looks like this...

$ ./configure && make
  CC       thirdparty/libtap_a-tap.o
  AR       libtap.a
  CC       watchman-argv.o
  CC       watchman-opt.o
  CC       watchman-cfg.o
  CC       watchman-log.o
  CC       watchman-json.o
  CC       watchman-bser.o
  CC       watchman-hash.o
  CC       watchman-ht.o
configure.ac: output extra build config summery to console

It's sometimes useful, and always harmless, to output some extra config
summery at the end of the configure script.  It'll save some debugging time
if your eye catches the -O2 in the CFLAGS before you set out to debug.
When that happened, you'd reconfigure with CFLAGS=-g and continue.

The output looks like this...

...
config.status: creating Makefile
config.status: creating thirdparty/jansson/jansson_config.h
config.status: creating config.h
config.status: config.h is unchanged
config.status: executing depfiles commands

Your build configuration:

        CPPFLAGS =  -D_REENTRANT -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE
        CFLAGS = -g -O2 -I/usr/local/Cellar/pcre/8.33/include -Wall -Wextra -Wdeclaration-after-statement -g -gdwarf-2 -fno-omit-frame-pointer
        LDFLAGS =
        prefix: /usr/local
        version: 2.9.5

$