tests: compile the C++ examples with warnings

[Akim Demaille]

commit 868ea55b7c2c557b4a7e767effa95451ccab3e87
Author: Akim Demaille <address@hidden>
Date:   Mon Nov 12 08:42:34 2018 +0100

    tests: compile the C++ examples with warnings
    
    * examples/c++/local.mk: Pass $(WARN_CXXFLAGS_TEST).

diff --git a/examples/c++/local.mk b/examples/c++/local.mk
index 654c2bd2..66556a70 100644
--- a/examples/c++/local.mk
+++ b/examples/c++/local.mk
@@ -32,7 +32,7 @@ if ENABLE_CXX14
   check_PROGRAMS += %D%/simple
   nodist_%C%_simple_SOURCES = $(simple_sources)
 
-  %C%_simple_CXXFLAGS = $(CXX11_CXXFLAGS)
+  %C%_simple_CXXFLAGS = $(CXX11_CXXFLAGS) $(WARN_CXXFLAGS_TEST)
   # Don't use gnulib's system headers.
   %C%_simple_CPPFLAGS = -I$(top_builddir)
   TESTS += %D%/simple.test
@@ -58,7 +58,7 @@ EXTRA_DIST += %D%/variant.test
 if ENABLE_CXX11
   check_PROGRAMS += %D%/variant-11
   nodist_%C%_variant_11_SOURCES = %D%/variant-11.yy
-  %C%_variant_11_CXXFLAGS = $(CXX11_CXXFLAGS)
+  %C%_variant_11_CXXFLAGS = $(CXX11_CXXFLAGS) $(WARN_CXXFLAGS_TEST)
   # Don't use gnulib's system headers.
   %C%_variant_11_CPPFLAGS = -I$(top_builddir)
   TESTS += %D%/variant-11.test