CMake: Solve compilation error with pre-compiled libraries and new GCC-6

Since version 6 G++ switched to C++11 by default, which breaks some logic
around WITH_CXX11 checks in out CMake files, leading to compilation errors.
This is easy to solve by explicitly enabling older C++ standard when C++11
was not explicitly enabled by CMake options.

However, G++-6 will also use new ABI by default even if older standard was
specified in the compiler options. This is being addressed by a special
define flag.

This tricks made it possible to use new G++-6 without need to recompile
any of pre-compiled libraries.

However, this might break compilation with existing system libraries, which
might already be using new ABI. We can't address this automatically, so
now we simply default WITH_C11 and WITH_CXX11 options to whatever defaults
of the current compiler are. This means, for G++-6 we'll set WITH_CXX11 to
truth. This should make linking with system libraries working just fine,
but to make pre-compiled libraries we still might need to disable CXX11.

This should work fine work for a new environments with G++-6 and install_deps
script run from scratch there, because C++ standard will be the same for
both Blender dependencies and Blender itself.

CMakeLists.txt

@@ -478,9 +478,19 @@ if(WIN32)
 endif()
 
 # Experimental support of C11 and C++11
-option(WITH_C11 "Build with C11 standard enabled, for development use only!" OFF)
+#
+# We default options to whatever default standard in the current compiler.
+if(CMAKE_COMPILER_IS_GNUCC AND (NOT "${CMAKE_C_COMPILER_VERSION}" VERSION_LESS "6.0") AND (NOT WITH_CXX11))
+	set(_c11_init ON)
+	set(_cxx11_init ON)
+else()
+	set(_c11_init OFF)
+	set(_cxx11_init OFF)
+endif()
+
+option(WITH_C11 "Build with C11 standard enabled, for development use only!" ${_c11_init})
 mark_as_advanced(WITH_C11)
-option(WITH_CXX11 "Build with C++11 standard enabled, for development use only!" OFF)
+option(WITH_CXX11 "Build with C++11 standard enabled, for development use only!" ${_cxx11_init})
 mark_as_advanced(WITH_CXX11)
 
 # Dependency graph
@@ -3024,12 +3034,22 @@ endif()
 
 if(WITH_CXX11)
 	if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_C_COMPILER_ID MATCHES "Clang")
+		# TODO(sergey): Do we want c++11 or gnu-c++11 here?
 		set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
 	elseif(MSVC12)
 		# Nothing special is needed, C++11 features are available by default.
 	else()
 		message(FATAL_ERROR "Compiler ${CMAKE_C_COMPILER_ID} is not supported for C++11 build yet")
 	endif()
+else()
+	# GCC-6 switched to C++11 by default, which would break linking with existing libraries
+	# by default. So we explicitly disable C++11 for a new GCC so no linking issues happens.
+	if(CMAKE_COMPILER_IS_GNUCC AND (NOT "${CMAKE_C_COMPILER_VERSION}" VERSION_LESS "6.0"))
+		set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++98")
+		# We also disable any of C++11 ABI from usage, so we wouldn't even try to
+		# link to stuff from std::__cxx11 namespace.
+		add_definitions("-D_GLIBCXX_USE_CXX11_ABI=0")
+	endif()
 endif()
 
 # Visual Studio has all standards it supports available by default

build_files/cmake/macros.cmake

@@ -803,7 +803,15 @@ macro(TEST_UNORDERED_MAP_SUPPORT)
 	#  UNORDERED_MAP_NAMESPACE, namespace for unordered_map, if found
 
 	include(CheckIncludeFileCXX)
-	CHECK_INCLUDE_FILE_CXX("unordered_map" HAVE_STD_UNORDERED_MAP_HEADER)
+
+	# Workaround for newer GCC (6.x+) where C++11 was enabled by default, which lead us
+	# to a situation when there is <unordered_map> include but which can't be used uless
+	# C++11 is enabled.
+	if(CMAKE_COMPILER_IS_GNUCC AND (NOT "${CMAKE_C_COMPILER_VERSION}" VERSION_LESS "6.0") AND (NOT WITH_CXX11))
+		set(HAVE_STD_UNORDERED_MAP_HEADER False)
+	else()
+		CHECK_INCLUDE_FILE_CXX("unordered_map" HAVE_STD_UNORDERED_MAP_HEADER)
+	endif()
 	if(HAVE_STD_UNORDERED_MAP_HEADER)
 		# Even so we've found unordered_map header file it doesn't
 		# mean unordered_map and unordered_set will be declared in