Allow overriding the AVX512 check with a NO_AVX512 define

cmake/system_check.cmake

@@ -109,6 +109,7 @@ else()
 endif()
 
 if (X86_64 OR X86)
+if (NOT NO_AVX512)
   file(WRITE ${PROJECT_BINARY_DIR}/avx512.c "#include <immintrin.h>\n\nint main(void){ __asm__ volatile(\"vbroadcastss -4 * 4(%rsi), %zmm2\"); }")
 execute_process(COMMAND ${CMAKE_C_COMPILER} -march=skylake-avx512 -c -v -o ${PROJECT_BINARY_DIR}/avx512.o ${PROJECT_BINARY_DIR}/avx512.c OUTPUT_QUIET ERROR_QUIET RESULT_VARIABLE NO_AVX512)
 if (NO_AVX512 EQUAL 1)
@@ -116,6 +117,7 @@ set (CCOMMON_OPT "${CCOMMON_OPT} -DNO_AVX512")
 endif()
   file(REMOVE "avx512.c" "avx512.o")
 endif()
+endif()
 
 include(CheckIncludeFile)
 CHECK_INCLUDE_FILE("stdatomic.h" HAVE_C11)