Fix build with GNU libstdc++ C++11 ABI

The GNU libstdc++ that ships witch gcc 5 can be used with the same ABI as
older versions, or with a new ABI which is conformant to the C++11 standard.
LyX did not build if the latter was used:
https://kojipkgs.fedoraproject.org//work/tasks/1267/9651267/build.log

This is now fixed by detecting the ABI version and disabling the wrong forward
declarations. At the same time, STD_STRING_USES_COW is switched off for the
C++11 ABI version, because the std::basic_string implementation is now C++11
conformant. Since the GNU libstdc++ can also used by other compilers such as
clang, we must not test for the compiler version.

config/lyxinclude.m4

@@ -159,6 +159,20 @@ AC_DEFUN([LYX_LIB_STDCXX],
 ])
 
 
+dnl Usage: LYX_LIB_STDCXX_CXX11_ABI: set lyx_cv_lib_stdcxx_cxx11_abi to yes
+dnl        if the STL library is GNU libstdc++ and the C++11 ABI is used.
+AC_DEFUN([LYX_LIB_STDCXX_CXX11_ABI],
+[AC_CACHE_CHECK([whether STL is libstdc++ using the C++11 ABI],
+               [lyx_cv_lib_stdcxx_cxx11_abi],
+[AC_TRY_COMPILE([#include<vector>], [
+#if ! defined(_GLIBCXX_USE_CXX11_ABI) || ! _GLIBCXX_USE_CXX11_ABI
+	    this is not libstdc++ using the C++11 ABI
+#endif
+],
+[lyx_cv_lib_stdcxx_cxx11_abi=yes], [lyx_cv_lib_stdcxx_cxx11_abi=no])])
+])
+
+
 AC_DEFUN([LYX_PROG_CXX],
 [AC_REQUIRE([AC_PROG_CXX])
 AC_REQUIRE([AC_PROG_CXXCPP])
@@ -166,10 +180,15 @@ AC_REQUIRE([AC_PROG_CXXCPP])
 AC_LANG_PUSH(C++)
 LYX_PROG_CLANG
 LYX_LIB_STDCXX
+LYX_LIB_STDCXX_CXX11_ABI
 AC_LANG_POP(C++)
 
 if test $lyx_cv_lib_stdcxx = "yes" ; then
-  AC_DEFINE(STD_STRING_USES_COW, 1, [std::string uses copy-on-write])
+  if test $lyx_cv_lib_stdcxx_cxx11_abi = "yes" ; then
+    AC_DEFINE(USE_GLIBCXX_CXX11_ABI, 1, [use GNU libstdc++ with C++11 ABI])
+  else
+    AC_DEFINE(STD_STRING_USES_COW, 1, [std::string uses copy-on-write])
+  fi
 else
   if test $lyx_cv_prog_clang = "yes" ; then
     AC_DEFINE(USE_LLVM_LIBCPP, 1, [use libc++ provided by llvm instead of GNU libstdc++])

development/cmake/ConfigureChecks.cmake

@@ -137,7 +137,20 @@ check_cxx_source_compiles(
 		return(0);
 	}
 	"
-STD_STRING_USES_COW)
+lyx_cv_lib_stdcxx)
+
+# Check whether STL is libstdc++ with C++11 ABI
+check_cxx_source_compiles(
+	"
+	#include <vector>
+	int main() {
+	#if ! defined(_GLIBCXX_USE_CXX11_ABI) || ! _GLIBCXX_USE_CXX11_ABI
+		this is not libstdc++ using the C++11 ABI
+	#endif
+		return(0);
+	}
+	"
+USE_GLIBCXX_CXX11_ABI)
 
 check_cxx_source_compiles(
 	"
@@ -151,7 +164,13 @@ check_cxx_source_compiles(
 lyx_cv_prog_clang)
 
 set(USE_LLVM_LIBCPP)
-if(NOT STD_STRING_USES_COW)
+set(STD_STRING_USES_COW)
+set(USE_GLIBCXX_CXX11_ABI)
+if(lyx_cv_lib_stdcxx)
+  if(NOT USE_GLIBCXX_CXX11_ABI)
+    set(STD_STRING_USES_COW 1)
+  endif()
+else()
   if(lyx_cv_prog_clang)
     # use libc++ provided by llvm instead of GNU libstdc++
     set(USE_LLVM_LIBCPP 1)

development/cmake/config.h.cmake

@@ -65,6 +65,9 @@
 // use libc++ provided by llvm instead of GNU libstdc++
 #cmakedefine USE_LLVM_LIBCPP 1
 
+// use GNU libstdc++ with C++11 ABI
+#cmakedefine USE_GLIBCXX_CXX11_ABI 1
+
 #cmakedefine Z_PREFIX 1
 
 ${Include_used_spellchecker}

src/support/strfwd.h

@@ -29,7 +29,10 @@ namespace lyx { typedef boost::uint32_t char_type; }
 #endif
 
 // Forward definitions do not work with libc++
-#ifdef USE_LLVM_LIBCPP
+// For gcc5 with the new std::string ABI forward declarations would work in
+// principle, but I am not sure whether we want non-standard
+// "namespace __cxx11" in our sources.
+#if defined(USE_LLVM_LIBCPP) || defined(USE_GLIBCXX_CXX11_ABI)
 #include <string>
 #else