Disable gcc-specific workaround for Clang to allow build with AVX512

There is currently a workaround for an issue in gcc that requires invoking gcc with the -fabi-version flag. This workaround is not needed for Clang and moreover is not supported.

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 62bf823..8a06873 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -232,7 +232,10 @@
 
   option(EIGEN_TEST_AVX512 "Enable/Disable AVX512 in tests/examples" OFF)
   if(EIGEN_TEST_AVX512)
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -mavx512f -fabi-version=6 -DEIGEN_ENABLE_AVX512")
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -mavx512f -DEIGEN_ENABLE_AVX512")
+    if (NOT "${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
+      set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fabi-version=6")
+    endif()
     message(STATUS "Enabling AVX512 in tests/examples")
   endif()