build: Make sure we don't redefine SCREENSHOTS but append to it

In 540741dd48d0720876a5e68b2aa4aa7a9d79b8a3 one screenshot was
moved into HAVE_CXX_11 guards to allow building with cxx11 disabled.

But in the case of cxx11 enabled we want all screenshots plus this
one so append here instead of redefine.

diff --git a/src/examples/Makefile.am b/src/examples/Makefile.am
index fa8257f..6cb703a 100644 (file)
--- a/src/examples/Makefile.am
+++ b/src/examples/Makefile.am
@@ -406,7 +406,7 @@ table_example_02:table_example_02.png:0.0 \
 menu_example_01:menu_example_01.png:0.5
 
 if HAVE_CXX11
-SCREENSHOTS = \
+SCREENSHOTS += \
 box_cxx_example_02:box_cxx_example_02.png:1.3
 endif