build: require gcc >= 5.3 and switch to C++14

Change-Id: If5d166e8679a19a30c5e98f12de348776f328186
Refs: #3076, #4462
diff --git a/wscript b/wscript
index 9107c75..20711d2 100644
--- a/wscript
+++ b/wscript
@@ -20,6 +20,8 @@
     conf.check_cfg(package='libndn-cxx', args=['--cflags', '--libs'],
                    uselib_store='NDN_CXX', mandatory=True)
 
+    conf.check_compiler_flags()
+
 def build(bld):
     bld.program(
         target='ndn-traffic',