Reduce build log verbosity

The default qmake build output is overly verbose. Adding the "silent"
switch to CONFIG makes it much more concise, comparable to default CMake
output. This way warnings and errors are much easier to find.

Adding CONFIG-=silent to the qmake command line doesn't restore the
verbosity because it is applied before the "CONFIG += silent" line in
goldendict.pro. Add a new CONFIG switch "verbose_build_output" to allow
increasing build log verbosity without editing goldendict.pro.

goldendict.pro

@@ -14,6 +14,14 @@ isEmpty( hasGit ) {
   system(echo $$VERSION > version.txt)
 }
 
+!CONFIG( verbose_build_output ) {
+  !win32|*-msvc* {
+    # Reduce build log verbosity except for MinGW builds (mingw-make cannot
+    # execute "@echo ..." commands inserted by qmake).
+    CONFIG += silent
+  }
+}
+
 # DEPENDPATH += . generators
 INCLUDEPATH += .