C library: getrandom requires glibc >= 2.25

As noticed in Homebrew/homebrew-core#102337,
which attempts to build on Ubuntu 16.04, sys/random.h isn't available
with glibc versions prior to 2.25. Therefore, replace the __linux__
inclusion check by one that tests for a particular glibc version.

Author: tautschnig

regression/cbmc-library/getrandom-01/main.c

@@ -1,8 +1,9 @@
-#ifdef __linux__
-#  include <sys/random.h>
-
 #  include <assert.h>
 
+#if defined(__GLIBC__) &&                                                      \
+  (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 25))
+#  include <sys/random.h>
+
 int main()
 {
   char zero_bytes[6] = {0};

src/ansi-c/library/random.c

@@ -1,6 +1,12 @@
 /* FUNCTION: getrandom */
 
-#ifdef __linux__
+#ifndef __CPROVER_ERRNO_H_INCLUDED
+#include <errno.h>
+#define __CPROVER_ERRNO_H_INCLUDED
+#endif
+
+#if defined(__GLIBC__) &&                                                      \
+  (__GLIBC__ > 2 || (__GLIBC__ == 2 && __GLIBC_MINOR__ >= 25))
 
 #  ifndef __CPROVER_SYS_RANDOM_H_INCLUDED
 #    include <sys/random.h>