CMakeLists.txt: explicitly set the standard to C++98

Set the C++ standard to C++98 to avoid GCC 6 compilation failures.

CMakeLists.txt

@@ -22,6 +22,16 @@ set(CMAKE_OSX_DEPLOYMENT_TARGET 10.7)
 # only relevant for building shared libs but let's set it regardless
 set(CMAKE_OSX_RPATH 1)
 
+# Set the C++ standard to C++98 to avoid compilation errors on GCC 6 (which
+# defaults to C++14).
+if(CMAKE_VERSION VERSION_LESS "3.1")
+  if(CMAKE_COMPILER_IS_GNUCXX)
+    set (CMAKE_CXX_FLAGS "--std=gnu++98 ${CMAKE_CXX_FLAGS}")
+  endif()
+else()
+  set (CMAKE_CXX_STANDARD 98)
+endif()
+
 project(SimGear)
 
 # read 'version' file into a variable (stripping any newlines or spaces)