Index: polly/trunk/cmake/polly_macros.cmake
===================================================================
--- polly/trunk/cmake/polly_macros.cmake
+++ polly/trunk/cmake/polly_macros.cmake
@@ -72,12 +72,17 @@
   endif()
 endmacro(add_polly_loadable_module)
 
-# # Use C99-compatible compile mode for all C source files of a target.
+# Use C99-compatible compile mode for all C source files of a target.
 function(target_enable_c99 _target)
   if(CMAKE_VERSION VERSION_GREATER "3.1")
     set_target_properties("${_target}" PROPERTIES C_STANDARD 99)
   elseif(CMAKE_COMPILER_IS_GNUCC)
     get_target_property(_sources "${_target}" SOURCES)
-    set_source_files_properties(${_sources} COMPILE_FLAGS "-std=gnu99")
+    foreach(_file IN LISTS _sources)
+      get_source_file_property(_lang "${_file}" LANGUAGE)
+      if(_lang STREQUAL "C")
+        set_source_files_properties(${_file} COMPILE_FLAGS "-std=gnu99")
+      endif()
+    endforeach()
   endif()
 endfunction()