CMake: explicitly require C++11 support

Explicitly require C++11 support to be able to compile terragear on
older systems.

CMakeLists.txt

@@ -6,6 +6,11 @@ include (CheckCXXSourceCompiles)
 include (CheckIncludeFile)
 include (CPack)
 
+# let's use & require C++11 - note these are only functional with CMake 3.1
+# we do manual fallbacks for CMake 3.0 in the compilers section
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED YES)
+
 project(TerraGear)
 
 # read 'version' file into a variable (stripping any newlines or spaces)
@@ -27,7 +32,6 @@ if(NOT CMAKE_BUILD_TYPE)
       FORCE)
 endif(NOT CMAKE_BUILD_TYPE)
 
-
 if (MSVC)
   GET_FILENAME_COMPONENT(PARENT_DIR ${PROJECT_SOURCE_DIR} PATH)
   if (CMAKE_CL_64)
@@ -126,6 +130,10 @@ set(PKGDATADIR ${CMAKE_INSTALL_PREFIX}/share/TerraGear)
 
 if(CMAKE_COMPILER_IS_GNUCXX)
     set(WARNING_FLAGS -Wall)
+
+    if (CMAKE_VERSION VERSION_LESS 3.1)
+       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+    endif()
 endif(CMAKE_COMPILER_IS_GNUCXX)
 
 if(WIN32)

CMakeModules/FindSimGear.cmake

@@ -187,6 +187,11 @@ unset(SIMGEAR_COMPILE_TEST CACHE)
 
 # disable OSG dependencies for test-compiling
 set(CMAKE_REQUIRED_DEFINITIONS "-DNO_OPENSCENEGRAPH_INTERFACE")
+
+set(CMAKE_CXX_STANDARD 11)
+set(CMAKE_CXX_STANDARD_REQUIRED YES)
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
+
 check_cxx_source_runs(
     "#include <cstdio>
     #include \"simgear/version.h\"