Fixed AVX512 detection with clang

Author: JohanMabille

include/xsimd/config/xsimd_instruction_set.hpp

@@ -106,7 +106,7 @@
 // AVX512 instructions are supported starting with gcc 6
 // see https://www.gnu.org/software/gcc/gcc-6/changes.html
 #if !defined(XSIMD_X86_INSTR_SET) && (defined(__AVX512__) || defined(__KNCNI__) || defined(__AVX512F__)\
-    && (!defined(__GNUC__) || __GNUC__ >= 6))
+    && (defined(__clang__) || (!defined(__GNUC__) || __GNUC__ >= 6)))
     #define XSIMD_X86_INSTR_SET XSIMD_X86_AVX512_VERSION
 
     #if defined(__AVX512VL__)