Bug 1390752 - Avoid std::basic_ios<char, std::char_traits<char>>::ope…

…rator bool() references. r=froydnj

This is similar to what we had until bug 1278456 removed them when we dropped
support for older libstdc++, but for a different symbol.

--HG--
extra : rebase_source : 641fc6c86c8f47e3dbd752bc20056f61646541a7

build/unix/stdc++compat/stdc++compat.cpp

@@ -160,3 +160,12 @@ namespace std {
 #endif
 }
 #endif
+
+#if MOZ_LIBSTDCXX_VERSION >= GLIBCXX_VERSION(3, 4, 21)
+namespace std
+{
+  /* Instantiate this template to avoid GLIBCXX_3.4.21 symbol versions
+   * depending on optimization level */
+  template basic_ios<char, char_traits<char> >::operator bool() const;
+}
+#endif