directfb-examples: explicitly add -lstdc++ for static build

Fixes [1] and [2]:
undefined reference to `___cxa_pure_virtual'
undefined reference to `___gxx_personality_v0'

[1] http://autobuild.buildroot.net/results/bd0/bd0f4b3e41bf6330f83e3d9b58d985cc63506a3b/
[2] http://autobuild.buildroot.net/results/584/584ace383204d97fcaad37742e2e035ca78a3d10/

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/directfb-examples/directfb-examples.mk

@@ -12,6 +12,10 @@ DIRECTFB_EXAMPLES_LICENSE_FILES = COPYING
 DIRECTFB_EXAMPLES_INSTALL_STAGING = YES
 DIRECTFB_EXAMPLES_DEPENDENCIES = directfb
 
+ifeq ($(BR2_STATIC_LIBS),y)
+        DIRECTFB_EXAMPLES_CONF_OPTS += LIBS=-lstdc++
+endif
+
 DIRECTFB_EXAMPLES_TARGETS_ =
 DIRECTFB_EXAMPLES_TARGETS_y =