diff -urN glibmm.orig/files/patch-glib_glibmm_ustring.cc glibmm/files/patch-glib_glibmm_ustring.cc
--- glibmm.orig/files/patch-glib_glibmm_ustring.cc	1970-01-01 01:00:00.000000000 +0100
+++ glibmm/files/patch-glib_glibmm_ustring.cc	2018-10-15 18:04:23.468373000 +0200
@@ -0,0 +1,29 @@
+--- glib/glibmm/ustring.cc.orig	2017-08-30 14:13:30 UTC
++++ glib/glibmm/ustring.cc
+@@ -1364,7 +1364,7 @@ ustring::FormatStream::to_string() const
+ #ifdef GLIBMM_HAVE_WIDE_STREAM
+   const std::wstring str = stream_.str();
+ 
+-#if defined(__STDC_ISO_10646__) && SIZEOF_WCHAR_T == 4
++#if (defined(__STDC_ISO_10646__) || defined(_LIBCPP_VERSION)) && SIZEOF_WCHAR_T == 4
+   // Avoid going through iconv if wchar_t always contains UCS-4.
+   glong n_bytes = 0;
+   const auto buf = make_unique_ptr_gfree(g_ucs4_to_utf8(
+@@ -1452,7 +1452,7 @@ operator>>(std::wistream& is, ustring& utf8_string)
+   std::wstring wstr;
+   is >> wstr;
+ 
+-#if defined(__STDC_ISO_10646__) && SIZEOF_WCHAR_T == 4
++#if (defined(__STDC_ISO_10646__) || defined(_LIBCPP_VERSION)) && SIZEOF_WCHAR_T == 4
+   // Avoid going through iconv if wchar_t always contains UCS-4.
+   glong n_bytes = 0;
+   const auto buf = make_unique_ptr_gfree(g_ucs4_to_utf8(
+@@ -1483,7 +1483,7 @@ operator<<(std::wostream& os, const ustring& utf8_stri
+ {
+   GError* error = nullptr;
+ 
+-#if defined(__STDC_ISO_10646__) && SIZEOF_WCHAR_T == 4
++#if (defined(__STDC_ISO_10646__) || defined(_LIBCPP_VERSION)) && SIZEOF_WCHAR_T == 4
+   // Avoid going through iconv if wchar_t always contains UCS-4.
+   const auto buf = make_unique_ptr_gfree(
+     g_utf8_to_ucs4(utf8_string.raw().data(), utf8_string.raw().size(), nullptr, nullptr, &error));