x11-toolkits/gtkmm20: unbreak build with Clang 6 (C++14 by default)

In file included from module.cc:3:
../../glib/glibmm/module.h:30:1: error: no member named 'string' in the global namespace; did you mean '::std::string'?
GTKMM_USING_STD(string)
^~~~~~~~~~~~~~~~~~~~~~~
../../glib/glibmmconfig.h:60:57: note: expanded from macro 'GTKMM_USING_STD'

1. define GTKMM_USING_STD(Symbol) namespace std { using ::Symbol; } ^~

/usr/include/c++/v1/iosfwd:194:65: note: '::std::string' declared here
typedef basic_string<char, char_traits<char>, allocator<char> > string;

^

class.cc:40:5: error: non-constant-expression cannot be narrowed from type 'guint'

  (aka 'unsigned int') to 'guint16' (aka 'unsigned short') in initializer list
  [-Wc++11-narrowing]
base_query.class_size,
^~~~~~~~~~~~~~~~~~~~~

class.cc:40:5: note: insert an explicit cast to silence this issue

base_query.class_size,
^~~~~~~~~~~~~~~~~~~~~
static_cast<guint16>()

[...]

Reported by: antoine (via bug 224669)
Obtained from: upstream (glibmm 2.31.2)
Suggested by: tijl (autoreconf)