[CMake][Polly] Remove dead CMake code

[tambre]

Summary

LLVM requires CMake 3.13.4 so remove code behind checks for an older version.

[tambre]

Simply code to use set_property(), which can handle multiple targets at once.

[phosek]

LGTM

polly/cmake/polly_macros.cmake

macro(add_polly_loadable_module name)
   endif()
   if (APPLE)
     # Darwin-specific linker flags for loadable modules.
     set_target_properties(${name} PROPERTIES
       LINK_FLAGS "-Wl,-flat_namespace -Wl,-undefined -Wl,suppress")
   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
 function(setup_polly_source_groups_helper pwd prefix glob_expr)
   file(GLOB children RELATIVE ${pwd} ${pwd}/*)
   foreach(child ${children})
     if (IS_DIRECTORY ${pwd}/${child})
       setup_polly_source_groups_helper(${pwd}/${child}

polly/lib/External/CMakeLists.txt

add_executable(polly-isl-test
     )
   set_target_properties(polly-isl-test PROPERTIES FOLDER "Polly")
 
   target_link_libraries(polly-isl-test PRIVATE
     PollyISL
     )
 
   # 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_target_properties(PollyISL PROPERTIES C_STANDARD 99)
+  set_target_properties(polly-isl-test PROPERTIES C_STANDARD 99)
 endif (POLLY_BUNDLED_ISL)
 
 set(PET_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/pet")
 set(PPCG_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/ppcg")
 set(PPCG_BINARY_DIR "${CMAKE_CURRENT_BINARY_DIR}/ppcg")
 
 # Determine version of ppcg
 if (EXISTS "${PPCG_SOURCE_DIR}/GIT_HEAD_ID")