Build the dummy plugin when BUILD_ALL_PLUGINS is set to 1.
Build the dummy plugin when BUILD_ALL_PLUGINS is set to 1.

Normal builds won't have that environment variable set, only
developer testing builds will attempt to build all plugins.
(cherry picked from commit 589b3ff6c28a80a21be4dd21dce74742cd39f9fd)

qtproject/qt-mobility,enthought/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,kaltsi/qt-mobility,qtproject/qt-mobility,qtproject/qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,tmcguire/qt-mobility,enthought/qt-mobility,KDE/android-qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,KDE/android-qt-mobility,enthought/qt-mobility,KDE/android-qt-mobility,kaltsi/qt-mobility,tmcguire/qt-mobility,enthought/qt-mobility,qtproject/qt-mobility,tmcguire/qt-mobility,enthought/qt-mobility,KDE/android-qt-mobility