Build X11 session management only if dependencies are found

This fixes the build breakage caused by Change I50b33d05 when attempting
to compile on Linux with session management enabled but libSM and/or
libICE not present.

Change-Id: I127f32e7041deb2ff968eae8458fee7d4e95d7a5
Reviewed-by: Samuel Gaist <samuel.gaist@edeltech.ch>
Reviewed-by: Bernd Weimer <bweimer@blackberry.com>
Reviewed-by: aavit <eirik.aavitsland@digia.com>

src/plugins/platforms/xcb/qxcbintegration.cpp

@@ -443,7 +443,7 @@ QByteArray QXcbIntegration::wmClass() const
     return m_wmClass;
 }
 
-#ifndef QT_NO_SESSIONMANAGER
+#if !defined(QT_NO_SESSIONMANAGER) && defined(XCB_USE_SM)
 QPlatformSessionManager *QXcbIntegration::createPlatformSessionManager(const QString &id, const QString &key) const
 {
     return new QXcbSessionManager(id, key);

src/plugins/platforms/xcb/qxcbintegration.h

@@ -101,7 +101,7 @@ public:
 
     QByteArray wmClass() const;
 
-#ifndef QT_NO_SESSIONMANAGER
+#if !defined(QT_NO_SESSIONMANAGER) && defined(XCB_USE_SM)
     QPlatformSessionManager *createPlatformSessionManager(const QString &id, const QString &key) const Q_DECL_OVERRIDE;
 #endif
 

src/plugins/platforms/xcb/xcb-plugin.pro

@@ -74,6 +74,7 @@ contains(QT_CONFIG, xcb-render) {
 
 # build with session management support
 contains(QT_CONFIG, xcb-sm) {
+    DEFINES += XCB_USE_SM
     LIBS += -lSM -lICE
     SOURCES += qxcbsessionmanager.cpp
     HEADERS += qxcbsessionmanager.h