Add CONFIG += silent to build to reduce log pollution

Qmake by default creates lots of unnecessary information during builds
which makes it hard to extract warnings and errors from the log.
Add the undocumented silent parameter to make it less talky.

config.pri

@@ -15,6 +15,9 @@ lessThan(QT_VER_MIN, 6):!CONFIG(no_opengl) {
   error ("You need at least Qt 5.6 to compile YACReader or YACReaderLibrary.")
   }
 
+# reduce log pollution
+CONFIG += silent
+
 # Disable coverflow for arm targets
 isEmpty(QMAKE_TARGET.arch) {
   QMAKE_TARGET.arch = $$QMAKE_HOST.arch