Utilize CMake's language standards abstraction when possible

Author: Chuck Atkins

tools/pybind11Tools.cmake

@@ -20,22 +20,32 @@ include(CMakeParseArguments)
 
 if(NOT PYBIND11_CPP_STANDARD AND NOT CMAKE_CXX_STANDARD)
   if(NOT MSVC)
-    check_cxx_compiler_flag("-std=c++14" HAS_CPP14_FLAG)
-
-    if (HAS_CPP14_FLAG)
-      set(PYBIND11_CPP_STANDARD -std=c++14)
+    if(CMAKE_CXX14_EXTENSION_COMPILE_OPTION)
+      set(PYBIND11_CPP_STANDARD ${CMAKE_CXX14_EXTENSION_COMPILE_OPTION})
+    elseif(CMAKE_CXX14_STANDARD_COMPILE_OPTION)
+      set(PYBIND11_CPP_STANDARD ${CMAKE_CXX14_STANDARD_COMPILE_OPTION})
+    elseif(CMAKE_CXX11_EXTENSION_COMPILE_OPTION)
+      set(PYBIND11_CPP_STANDARD ${CMAKE_CXX11_EXTENSION_COMPILE_OPTION})
+    elseif(CMAKE_CXX11_STANDARD_COMPILE_OPTION)
+      set(PYBIND11_CPP_STANDARD ${CMAKE_CXX11_STANDARD_COMPILE_OPTION})
     else()
-      check_cxx_compiler_flag("-std=c++11" HAS_CPP11_FLAG)
-      if (HAS_CPP11_FLAG)
-        set(PYBIND11_CPP_STANDARD -std=c++11)
+      check_cxx_compiler_flag("-std=c++14" HAS_CPP14_FLAG)
+      if(HAS_CPP14_FLAG)
+        set(PYBIND11_CPP_STANDARD -std=c++14)
       else()
-        message(FATAL_ERROR "Unsupported compiler -- pybind11 requires C++11 support!")
+        check_cxx_compiler_flag("-std=c++11" HAS_CPP11_FLAG)
+        if (HAS_CPP11_FLAG)
+          set(PYBIND11_CPP_STANDARD -std=c++11)
+        endif()
       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()