message: qt4 compatibility, remove png library requirement
diff: @@ -3,7 +3,8 @@ TARGET = peercoin-qt
  macx:TARGET = "Peercoin-Qt"
  VERSION = 0.6.0
  INCLUDEPATH += src src/json src/qt
  -QT += network core widgets
  +QT += network core
  +greaterThan(QT_MAJOR_VERSION, 4):QT+=widgets
  DEFINES += QT_GUI BOOST_THREAD_USE_LIB BOOST_SPIRIT_THREADSAFE
  CONFIG += no_include_pwd
  CONFIG += thread
  @@ -54,7 +55,7 @@ win32:QMAKE_LFLAGS *= -Wl,--large-address-aware
  contains(USE_QRCODE, 1) {
  message(Building with QRCode support)
  DEFINES += USE_QRCODE
  - LIBS += -lqrencode -lpng
  + LIBS += -lqrencode
  }
  # use: qmake "USE_UPNP=1" ( enabled by default; default)
  