Utilize CMake's language standards abstraction when possible

Author: Chuck Atkins

tools/pybind11Tools.cmake

@@ -18,24 +18,40 @@ find_package(PythonLibsNew ${PYBIND11_PYTHON_VERSION} REQUIRED)
 include(CheckCXXCompilerFlag)
 include(CMakeParseArguments)
 
+# Use the language standards abstraction if CMake supports it with the current compiler
+if(NOT CMAKE_VERSION VERSION_LESS 3.1)
+  if(NOT CMAKE_CXX_STANDARD)
+    if(CMAKE_CXX14_STANDARD_COMPILE_OPTION)
+      set(CMAKE_CXX_STANDARD 14)
+    elseif(CMAKE_CXX11_STANDARD_COMPILE_OPTION)
+      set(CMAKE_CXX_STANDARD 11)
+    endif()
+  endif()
+  if(CMAKE_CXX_STANDARD)
+    set(CMAKE_CXX_EXTENSIONS OFF)
+    set(CMAKE_CXX_STANDARD_REQUIRED ON)
+  endif()
+endif()
+
+# Fall back to heuristics 
 if(NOT PYBIND11_CPP_STANDARD AND NOT CMAKE_CXX_STANDARD)
-  if(NOT MSVC)
+  if(MSVC)
+    set(PYBIND11_CPP_STANDARD /std:c++14)
+  else()
     check_cxx_compiler_flag("-std=c++14" HAS_CPP14_FLAG)
-
-    if (HAS_CPP14_FLAG)
+    if(HAS_CPP14_FLAG)
       set(PYBIND11_CPP_STANDARD -std=c++14)
     else()
       check_cxx_compiler_flag("-std=c++11" HAS_CPP11_FLAG)
-      if (HAS_CPP11_FLAG)
+      if(HAS_CPP11_FLAG)
         set(PYBIND11_CPP_STANDARD -std=c++11)
-      else()
-        message(FATAL_ERROR "Unsupported compiler -- pybind11 requires C++11 support!")
       endif()
     endif()
-  elseif(MSVC)
-    set(PYBIND11_CPP_STANDARD /std:c++14)
   endif()
 
+  if(NOT PYBIND11_CPP_STANDARD)
+    message(FATAL_ERROR "Unsupported compiler -- pybind11 requires C++11 support!")
+  endif()
   set(PYBIND11_CPP_STANDARD ${PYBIND11_CPP_STANDARD} CACHE STRING
       "C++ standard flag, e.g. -std=c++11, -std=c++14, /std:c++14.  Defaults to C++14 mode." FORCE)
 endif()