Make it possible to override IDE_BUILD_TREE.
Make it possible to override IDE_BUILD_TREE.

This is helpfull for building plugins from outside
the QtCreator source tree.

dmik/qt-creator-os2,maui-packages/qt-creator,KDAB/KDAB-Creator,dmik/qt-creator-os2,omniacreator/qtcreator,ostash/qt-creator-i18n-uk,richardmg/qtcreator,farseerri/git_code,omniacreator/qtcreator,renatofilho/QtCreator,danimo/qt-creator,bakaiadam/collaborative_qt_creator,xianian/qt-creator,amyvmiwei/qt-creator,amyvmiwei/qt-creator,azat/qtcreator,enricoros/k-qt-creator-inspector,KDE/android-qt-creator,xianian/qt-creator,malikcjm/qtcreator,AltarBeastiful/qt-creator,KDE/android-qt-creator,enricoros/k-qt-creator-inspector,amyvmiwei/qt-creator,farseerri/git_code,ostash/qt-creator-i18n-uk,xianian/qt-creator,darksylinc/qt-creator,enricoros/k-qt-creator-inspector,hdweiss/qt-creator-visualizer,kuba1/qtcreator,Distrotech/qtcreator,richardmg/qtcreator,Distrotech/qtcreator,bakaiadam/collaborative_qt_creator,danimo/qt-creator,darksylinc/qt-creator,Distrotech/qtcreator,martyone/sailfish-qtcreator,malikcjm/qtcreator,azat/qtcreator,darksylinc/qt-creator,sandsmark/qtcreator-minimap,xianian/qt-creator,duythanhphan/qt-creator,colede/qtcreator,KDAB/KDAB-Creator,yinyunqiao/qtcreator,renatofilho/QtCreator,colede/qtcreator,danimo/qt-creator,xianian/qt-creator,dmik/qt-creator-os2,kuba1/qtcreator,syntheticpp/qt-creator,maui-packages/qt-creator,syntheticpp/qt-creator,richardmg/qtcreator,colede/qtcreator,farseerri/git_code,amyvmiwei/qt-creator,hdweiss/qt-creator-visualizer,ostash/qt-creator-i18n-uk,farseerri/git_code,xianian/qt-creator,Distrotech/qtcreator,darksylinc/qt-creator,sandsmark/qtcreator-minimap,yinyunqiao/qtcreator,duythanhphan/qt-creator,malikcjm/qtcreator,kuba1/qtcreator,sandsmark/qtcreator-minimap,kuba1/qtcreator,danimo/qt-creator,martyone/sailfish-qtcreator,syntheticpp/qt-creator,martyone/sailfish-qtcreator,hdweiss/qt-creator-visualizer,colede/qtcreator,pcacjr/qt-creator,yinyunqiao/qtcreator,jonnor/qt-creator,darksylinc/qt-creator,KDE/android-qt-creator,malikcjm/qtcreator,duythanhphan/qt-creator,darksylinc/qt-creator,farseerri/git_code,xianian/qt-creator,KDE/android-qt-creator,yinyunqiao/qtcreator,AltarBeastiful/qt-creator,farseerri/git_code,duythanhphan/qt-creator,omniacreator/qtcreator,martyone/sailfish-qtcreator,maui-packages/qt-creator,yinyunqiao/qtcreator,pcacjr/qt-creator,AltarBeastiful/qt-creator,AltarBeastiful/qt-creator,martyone/sailfish-qtcreator,AltarBeastiful/qt-creator,bakaiadam/collaborative_qt_creator,amyvmiwei/qt-creator,renatofilho/QtCreator,KDAB/KDAB-Creator,richardmg/qtcreator,jonnor/qt-creator,maui-packages/qt-creator,Distrotech/qtcreator,ostash/qt-creator-i18n-uk,renatofilho/QtCreator,dmik/qt-creator-os2,enricoros/k-qt-creator-inspector,danimo/qt-creator,sandsmark/qtcreator-minimap,kuba1/qtcreator,darksylinc/qt-creator,azat/qtcreator,bakaiadam/collaborative_qt_creator,malikcjm/qtcreator,syntheticpp/qt-creator,omniacreator/qtcreator,farseerri/git_code,syntheticpp/qt-creator,danimo/qt-creator,malikcjm/qtcreator,omniacreator/qtcreator,yinyunqiao/qtcreator,martyone/sailfish-qtcreator,sandsmark/qtcreator-minimap,colede/qtcreator,hdweiss/qt-creator-visualizer,amyvmiwei/qt-creator,colede/qtcreator,ostash/qt-creator-i18n-uk,AltarBeastiful/qt-creator,azat/qtcreator,amyvmiwei/qt-creator,jonnor/qt-creator,danimo/qt-creator,darksylinc/qt-creator,kuba1/qtcreator,richardmg/qtcreator,pcacjr/qt-creator,azat/qtcreator,yinyunqiao/qtcreator,duythanhphan/qt-creator,jonnor/qt-creator,danimo/qt-creator,Distrotech/qtcreator,KDE/android-qt-creator,kuba1/qtcreator,martyone/sailfish-qtcreator,sandsmark/qtcreator-minimap,duythanhphan/qt-creator,dmik/qt-creator-os2,maui-packages/qt-creator,enricoros/k-qt-creator-inspector,bakaiadam/collaborative_qt_creator,ostash/qt-creator-i18n-uk,duythanhphan/qt-creator,KDE/android-qt-creator,richardmg/qtcreator,colede/qtcreator,maui-packages/qt-creator,martyone/sailfish-qtcreator,renatofilho/QtCreator,KDE/android-qt-creator,enricoros/k-qt-creator-inspector,xianian/qt-creator,martyone/sailfish-qtcreator,hdweiss/qt-creator-visualizer,jonnor/qt-creator,KDE/android-qt-creator,AltarBeastiful/qt-creator,maui-packages/qt-creator,dmik/qt-creator-os2,jonnor/qt-creator,pcacjr/qt-creator,azat/qtcreator,farseerri/git_code,pcacjr/qt-creator,KDAB/KDAB-Creator,AltarBeastiful/qt-creator,enricoros/k-qt-creator-inspector,dmik/qt-creator-os2,xianian/qt-creator,KDAB/KDAB-Creator,syntheticpp/qt-creator,kuba1/qtcreator,richardmg/qtcreator,Distrotech/qtcreator,omniacreator/qtcreator,kuba1/qtcreator,danimo/qt-creator,amyvmiwei/qt-creator,bakaiadam/collaborative_qt_creator,hdweiss/qt-creator-visualizer,ostash/qt-creator-i18n-uk,pcacjr/qt-creator,bakaiadam/collaborative_qt_creator,malikcjm/qtcreator,renatofilho/QtCreator,syntheticpp/qt-creator,omniacreator/qtcreator,pcacjr/qt-creator,KDAB/KDAB-Creator