Fixed building --with-cpp_test_module on some systems.

auto/modules

diff --git a/auto/modules b/auto/modules
index dab660f..7d3216b 100644
--- a/auto/modules
+++ b/auto/modules
@@ -458,6 +458,7 @@
 
 if [ $NGX_CPP_TEST = YES ]; then
     NGX_MISC_SRCS="$NGX_MISC_SRCS $NGX_CPP_TEST_SRCS"
+    CORE_LIBS="$CORE_LIBS -lstdc++"
 fi