bug#65276: Build failure with gcc < 4.8

[Bruno Haible]

Building the current coreutils on GNU/kFreeBSD 7, I get link errors:

  CCLD     src/cksum
src/cksum-cksum.o: In function `pclmul_supported':
/home/bruno/coreutils-2023-08-13/build-64/../src/cksum.c:149: undefined 
reference to `__builtin_cpu_supports'
/home/bruno/coreutils-2023-08-13/build-64/../src/cksum.c:150: undefined 
reference to `__builtin_cpu_supports'
collect2: error: ld returned 1 exit status
make[2]: *** [src/cksum] Error 1
  CCLD     src/wc
src/wc.o: In function `avx2_supported':
/home/bruno/coreutils-2023-08-13/build-64/../src/wc.c:150: undefined reference 
to `__builtin_cpu_supports'
collect2: error: ld returned 1 exit status
make[2]: *** [src/wc] Error 1

The reason is that the __builtin_cpu_supports function does not exist
(since the gcc version is 4.7.2 and __builtin_cpu_supports was only
introduced in gcc 4.8), but the configure test succeeds: Compiling this file
=========================== foo.c ===========================
#include <x86intrin.h>

int
main (void)
{
  return __builtin_cpu_supports ("pclmul");
}
=============================================================
merely produces warnings:
$ gcc -c -Wall foo.c
foo.c: In function ‘main’:
foo.c:6:3: warning: implicit declaration of function ‘__builtin_cpu_supports’ 
[-Wimplicit-function-declaration]

The attached patch fixes it.

0001-cksum-wc-Fix-link-errors-with-gcc-4.8.patch
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