Add Qt version check at build time

vlmc.pro

+!contains(QT_MAJOR_VERSION, 4) {
+    error( "Unsupported Qt version $$QT_VERSION found. Requires at least Qt 4.5" )
+}
+
+!contains( QT_MINOR_VERSION, 5 ) {
+    !contains( QT_MINOR_VERSION, 6 ) {
+        error( "Unsupported Qt version $$QT_VERSION found. Requires at least Qt 4.5" )
+    }
+}
+
 TEMPLATE = subdirs
 SUBDIRS = src \
             launcher