provide getrandom wrapper to support glibc < 2.25

Debian stretch (currently stable) only has glibc 2.24...

random.c

@@ -1,11 +1,21 @@
 #include <errno.h>
 #include <string.h>
 
-#include <sys/random.h>
-
 #include "random.h"
 #include "util.h"
 
+#if __has_include(<sys/random.h>)
+// glibc 2.25 and later
+#include <sys/random.h>
+#else
+#include <unistd.h>
+#include <sys/syscall.h>
+
+static ssize_t getrandom(void *buf, size_t buflen, unsigned int flags) {
+    return syscall(SYS_getrandom, buf, buflen, flags);
+}
+#endif
+
 void get_random_seed(void *buf, size_t size) {
     while (size > 0) {
         ssize_t r;