Build fails

Building from git master (04c7ec96):

  CXX      libxfce4_pp_la-dummy.lo
In file included from ../xfce4++/util/fixes.h:36,
                 from dummy.cc:22:
/usr/local/include/xfce4/libxfce4util/libxfce4util-config.h:23:2: error: #error "Only <libxfce4util/libxfce4util.h> can be included directly, this file may disappear or change contents"
   23 | #error "Only <libxfce4util/libxfce4util.h> can be included directly, this file may disappear or change contents"
      |  ^~~~~
In file included from ../xfce4++/util/fixes.h:38:
/usr/local/include/xfce4/libxfce4util/xfce-gio-extensions.h:21:2: error: #error "Only <libxfce4util/libxfce4util.h> can be included directly, this file may disappear or change contents"
   21 | #error "Only <libxfce4util/libxfce4util.h> can be included directly, this file may disappear or change contents"
      |  ^~~~~
In file included from ../xfce4++/util/fixes.h:54:
/usr/local/include/xfce4/libxfce4util/xfce-i18n.h:24:2: error: #error "Only <libxfce4util/libxfce4util.h> can be included directly, this file may disappear or change contents"
   24 | #error "Only <libxfce4util/libxfce4util.h> can be included directly, this file may disappear or change contents"
      |  ^~~~~

See also xfce4-cpufreq-plugin#18 (closed).