Fix configure check for __builtin_bswap64.

* configure.ac: Use AC_TRY_LINK rather than AC_TRY_COMPILE to
check for __builtin_bswap64. Since calling an non-existing
function typically results in a warning only at compile time, but
fails at link time. Patch contributed by by George Koehler.

ChangeLog

+2019-12-05  Niels Möller  <nisse@lysator.liu.se>
+
+	* configure.ac: Use AC_TRY_LINK rather than AC_TRY_COMPILE to
+	check for __builtin_bswap64. Since calling an non-existing
+	function typically results in a warning only at compile time, but
+	fails at link time. Patch contributed by by George Koehler.
+
 2019-12-04  Niels Möller  <nisse@lysator.liu.se>
 
 	* testsuite/testutils.c (test_cipher_cfb8): Add cast of size_t to

configure.ac

@@ -213,7 +213,7 @@ AC_C_BIGENDIAN([AC_DEFINE([WORDS_BIGENDIAN], 1)
 
 AC_CACHE_CHECK([for __builtin_bswap64],
 		nettle_cv_c_builtin_bswap64,
-[AC_TRY_COMPILE([
+[AC_TRY_LINK([
 #include <stdint.h>
 ],[
 uint64_t x = 17;