[LyX/2.3-staging] Fix compiler warnings

[Georg Baum]

commit 21758ae4ab52444513e470fece0d32931016e610
Author: Georg Baum <b...@lyx.org>
Date:   Sun May 8 12:25:09 2016 +0200

    Fix compiler warnings

diff --git a/src/frontends/qt4/GuiApplication.cpp 
b/src/frontends/qt4/GuiApplication.cpp
index 4bc69a3..163cb63 100644
--- a/src/frontends/qt4/GuiApplication.cpp
+++ b/src/frontends/qt4/GuiApplication.cpp
@@ -750,8 +750,8 @@ class QWindowsMimeMetafile : public QWINDOWSMIME {
 public:
        QWindowsMimeMetafile() {}
 
-       bool canConvertFromMime(FORMATETC const & formatetc,
-               QMimeData const * mimedata) const
+       bool canConvertFromMime(FORMATETC const & /*formatetc*/,
+               QMimeData const * /*mimedata*/) const
        {
                return false;
        }
@@ -765,14 +765,14 @@ public:
                return pDataObj->QueryGetData(&formatetc) == S_OK;
        }
 
-       bool convertFromMime(FORMATETC const & formatetc,
-               const QMimeData * mimedata, STGMEDIUM * pmedium) const
+       bool convertFromMime(FORMATETC const & /*formatetc*/,
+               const QMimeData * /*mimedata*/, STGMEDIUM * /*pmedium*/) const
        {
                return false;
        }
 
        QVariant convertToMime(QString const & mimetype, IDataObject * pDataObj,
-               QVariant::Type preferredType) const
+               QVariant::Type /*preferredType*/) const
        {
                QByteArray data;
                if (!canConvertToMime(mimetype, pDataObj))
@@ -803,7 +803,7 @@ public:
 
 
        QVector<FORMATETC> formatsForMime(QString const & mimetype,
-               QMimeData const * mimedata) const
+               QMimeData const * /*mimedata*/) const
        {
                QVector<FORMATETC> formats;
                if (mimetype == emfMimeType() || mimetype == wmfMimeType())