use #ifdef QT_UI to distinguish Qt UI instead of hardcoded #if 0

bitcoin-qt.pro

@@ -2,6 +2,7 @@ TEMPLATE = app
 TARGET =
 DEPENDPATH += .
 INCLUDEPATH += src src/json src/cryptopp src/qt
+DEFINES += QT_GUI
 
 # for boost 1.37, add -mt to the boost libraries
 unix:LIBS += -lssl -lcrypto -lboost_system -lboost_filesystem -lboost_program_options -lboost_thread -ldb_cxx
@@ -66,7 +67,7 @@ HEADERS += src/qt/bitcoingui.h \
     src/qt/guiconstants.h \
     src/qt/optionsmodel.h \
     src/qt/monitoreddatamapper.h \
-    src/externui.h \
+    src/qtui.h \
     src/qt/transactiondesc.h \
     src/qt/transactiondescdialog.h \
     src/qt/bitcoinamountfield.h