diff --git a/polly/cmake/polly_macros.cmake b/polly/cmake/polly_macros.cmake
--- a/polly/cmake/polly_macros.cmake
+++ b/polly/cmake/polly_macros.cmake
@@ -72,21 +72,6 @@
   endif()
 endmacro(add_polly_loadable_module)
 
-# 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)
-    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()
-
 # Recursive helper for setup_source_group. Traverse the file system and add
 # source files matching the glob_expr to the prefix, recursing into
 # subdirectories as they are encountered
diff --git a/polly/lib/External/CMakeLists.txt b/polly/lib/External/CMakeLists.txt
--- a/polly/lib/External/CMakeLists.txt
+++ b/polly/lib/External/CMakeLists.txt
@@ -293,8 +293,7 @@
     )
 
   # ISL requires at least C99 to compile. gcc < 5.0 use -std=gnu89 as default.
-  target_enable_c99(PollyISL)
-  target_enable_c99(polly-isl-test)
+  set_property(TARGET PollyISL polly-isl-test PROPERTY C_STANDARD 99)
 endif (POLLY_BUNDLED_ISL)
 
 set(PET_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/pet")