build: make the build process less verbose.

configure.ac

@@ -7,6 +7,7 @@ dnl  We cannot use subdir-objects because of a bug that will eg. remove
 dnl  ../src/.deps upon distclean when a file from ../src/ is used.
 AM_INIT_AUTOMAKE([no-define tar-ustar silent-rules])
 dnl AM_MAINTAINER_MODE
+AM_DEFAULT_VERBOSITY=0
 
 AM_CONFIG_HEADER(config.h)