CppunitTest_cppcanvas_emfplus: disable this in case there is no display to use

This test is already disabled on non-Linux, but it fails in case there
is no display to use. For now just disable it in that case.

Change-Id: I29c52e803a1fca5f2bdeeb655c573ad8fef622e8

cppcanvas/Module_cppcanvas.mk

@@ -30,9 +30,11 @@ $(eval $(call gb_Module_add_check_targets,cppcanvas,\
     CppunitTest_cppcanvas_test \
 ))
 
+ifneq ($(DISPLAY),)
 $(eval $(call gb_Module_add_slowcheck_targets,cppcanvas,\
     CppunitTest_cppcanvas_emfplus \
 ))
 endif
+endif
 
 # vim: set noet sw=4 ts=4: