util: add support for getrandom()

Add support for the Linux getrandom() system call, which is available
in glibc since 2.25.

configure

@@ -633,6 +633,11 @@ if test_code 'arc4random_buf()' 'stdlib.h' '' '' 'arc4random_buf(NULL, 0);'; the
   add_def HAVE_ARC4RANDOM
 fi
 
+if test_code 'getrandom()' 'stdlib.h sys/random.h' '' '' \
+    'getrandom(NULL, 256, 0);'; then
+  add_def HAVE_GETRANDOM
+fi
+
 RECVMMSG_CODE='
   struct mmsghdr hdr;
   return !recvmmsg(0, &hdr, 1, MSG_DONTWAIT, 0);'