fix build script to specify cxxflags instead of cflags

wscript

@@ -8,4 +8,4 @@ def build(bld):
     bld(features = "cxx cxxprogram",
         target = "svi",
         source = bld.path.ant_glob("src/**/*.cc"),
-        cflags = ["-Wall"])
+        cxxflags = ["-Wall", "-std=gnu++14"])