Created attachment 4031 
Proposed patch

Hi,

The build of 2.99.0 fails at linking time here, for the gtk-3.0 engine:
  CCLD   libxfce.la
.libs/libxfce_la-xfce_engine.o: In function `render_line':
/«PKGBUILDDIR»/gtk-3.0/xfce_engine.c:370: undefined reference to `floor'
.libs/libxfce_la-xfce_engine.o: In function `render_frame':
/«PKGBUILDDIR»/gtk-3.0/xfce_engine.c:460: undefined reference to `floor'
/«PKGBUILDDIR»/gtk-3.0/xfce_engine.c:461: undefined reference to `ceil'

The attached patch seems to fix the issue for me.
Thanks.