Compile Issue in platformsupport of Qt5 due to non-standard locations for EGL headers

eglconvenience.pri requires "CONFIG += egl", i.e.:

contains(QT_CONFIG,egl) {
    ...

    CONFIG += egl
}

To find the correct paths for EGL headers to build successfully, in the case where EGL headers are not in standard locations (such as '/usr/include' of the target filesystem).