Apply c++ standard flag only to files of CXX language. (#1678)

tools/pybind11Config.cmake.in

@@ -90,7 +90,11 @@ if(NOT TARGET ${PN}::pybind11)
       set_property(TARGET ${PN}::module APPEND PROPERTY INTERFACE_LINK_LIBRARIES ${PYTHON_LIBRARIES})
     endif()
 
-    set_property(TARGET ${PN}::pybind11 APPEND PROPERTY INTERFACE_COMPILE_OPTIONS "${PYBIND11_CPP_STANDARD}")
+    if(CMAKE_VERSION VERSION_LESS 3.3)
+      set_property(TARGET ${PN}::pybind11 APPEND PROPERTY INTERFACE_COMPILE_OPTIONS "${PYBIND11_CPP_STANDARD}")
+    else()
+      set_property(TARGET ${PN}::pybind11 APPEND PROPERTY INTERFACE_COMPILE_OPTIONS $<$<COMPILE_LANGUAGE:CXX>:${PYBIND11_CPP_STANDARD}>)
+    endif()
 
     get_property(_iid TARGET ${PN}::pybind11 PROPERTY INTERFACE_INCLUDE_DIRECTORIES)
     get_property(_ill TARGET ${PN}::module PROPERTY INTERFACE_LINK_LIBRARIES)

tools/pybind11Tools.cmake

@@ -185,7 +185,11 @@ function(pybind11_add_module target_name)
   endif()
 
   # Make sure C++11/14 are enabled
-  target_compile_options(${target_name} PUBLIC ${PYBIND11_CPP_STANDARD})
+  if(CMAKE_VERSION VERSION_LESS 3.3)
+    target_compile_options(${target_name} PUBLIC ${PYBIND11_CPP_STANDARD})
+  else()
+    target_compile_options(${target_name} PUBLIC $<$<COMPILE_LANGUAGE:CXX>:${PYBIND11_CPP_STANDARD}>)
+  endif()
 
   if(ARG_NO_EXTRAS)
     return()