Index: cmake/polly_macros.cmake
===================================================================
--- cmake/polly_macros.cmake
+++ cmake/polly_macros.cmake
@@ -76,7 +76,7 @@
 function(target_enable_c99 _target)
   if(CMAKE_VERSION VERSION_GREATER "3.1")
     set_target_properties("${_target}" PROPERTIES C_STANDARD 99)
-  elseif(CMAKE_COMPILER_IS_GNUCC)
+  elseif(CMAKE_COMPILER_IS_GNUCC OR ("${CMAKE_C_COMPILER_ID}" STREQUAL "Clang"))
     get_target_property(_sources "${_target}" SOURCES)
     foreach(_file IN LISTS _sources)
       get_source_file_property(_lang "${_file}" LANGUAGE)