xfce4-panel needs a symbol from libm, but only indrectly links to it. It should explicitly link to it. 

See: https://fedoraproject.org/wiki/Features/ChangeInImplicitDSOLinking
for more info. 

/usr/bin/ld: xfce4_panel-panel-properties.o: undefined reference to symbol 'rint@@GLIBC_2.2.5'
/usr/bin/ld: note: 'rint@@GLIBC_2.2.5' is defined in DSO /lib64/libm.so.6 so try adding it to the linker command line

hacky patch: 

diff -Nur xfce4-panel-4.6.3.orig/panel/Makefile.in xfce4-panel-4.6.3/panel/Makefile.in
--- xfce4-panel-4.6.3.orig/panel/Makefile.in	2010-02-13 19:51:28.000000000 -0700
+++ xfce4-panel-4.6.3/panel/Makefile.in	2010-02-13 20:49:00.000000000 -0700
@@ -325,7 +325,7 @@
 	$(LIBXFCEGUI4_LIBS)							\
 	$(LIBSTARTUP_NOTIFICATION_LIBS)						\
 	$(GMODULE_LIBS)								\
-	$(LIBX11_LIBS)
+	$(LIBX11_LIBS) -lm
 
 xfce4_panel_DEPENDENCIES = \
 	$(top_builddir)/libxfce4panel/libxfce4panel.la