编译gnutls3.5.3，make时报错：

...
rnd-linux.c: In function 'have_getrandom':
rnd-linux.c:59:42: error: 'SYS_getrandom' undeclared (first use in this function)
...

参考：https://lists.gnupg.org/pipermail/gnutls-devel/2016-August/008126

** libgnutls: Use getrandom where available via the syscall interface.
This works around an issue of not-using getrandom even if it exists
since glibc doesn’t declare such function.

参考：http://gnutls.org/manual/gnutls

GnuTLS requires the following system calls to be available for its proper operation.
    nanosleep
    time
    gettimeofday
    clock_gettime
    getrusage
    getpid
    send
    recv
    sendmsg
    read (to read from /dev/urandom)
    getrandom (this is Linux-kernel specific)
    poll 

参考：https://github.com/aria2/aria2/issues/346
参考：https://github.com/aria2/aria2/commit/fd6f5ebc32eb5f74501e11b7e519642ee95d49f0

 AC_MSG_CHECKING([for getrandom linux syscall interface])
 AC_LINK_IFELSE([AC_LANG_PROGRAM([[
+#include <sys/syscall.h>
 #include <linux/random.h>
 ]],
 [[
 int x = GRND_NONBLOCK;
+int y = (int)SYS_getrandom;
 ]])],
   [have_getrandom_interface=yes
    AC_DEFINE([HAVE_GETRANDOM_INTERFACE], [1], [Define to 1 if getrandom linux syscall interface is available.])],

参考：https://lists.kernelnewbies.org/pipermail/kernelnewbies/2016-June/016461

参考：https://bugs.ruby-lang.org/issues/11395
参考：https://sourceforge.net/p/ltp/mailman/message/34238553/
参考：http://stackoverflow.com/questions/36744009/checking-for-getrandom-function-returns-no-when-building-python-3-5-1-on-ubunt
参考：https://github.com/http2/http2-spec/wiki/ALPN-Status
参考：http://homeunix.nl/newsportal/article.php?id=1446222&group=linux.debian.bugs.dist#1446222

参考：https://github.com/rbenv/ruby-build/issues/914
参考：http://stackoverflow.com/questions/36744009/checking-for-getrandom-function-returns-no-when-building-python-3-5-1-on-ubunt

The build fails because it cannot find the getrandom syscall. The getrandom call was introduced in version 3.17 of the Linux kernel (according to its man page). 

那么是Linux内核导致的问题？

但是经过测试：
1. Linux 3.10.0-327.28.2.el7.x86_64 不行
2. Linux 3.18.40 不行
3. Linux 4.7.0-1.el7.elrepo.x86_64 不行

那么原因应该不仅仅是Linux内核版本的问题。

参考：

怎样升级Linux内核并开启BBR

怎样编译安装glibc

怎么解决？

最后是通过gnutls官方提供的git途径编译安装gnutls成功。中途没有出现SYS_getrandom未声明这样的报错。

经过测试：
1. Linux 3.10.0-327.28.2.el7.x86_64 尚未测试
2. Linux 3.18.40 编译安装成功
3. Linux 4.7.0-1.el7.elrepo.x86_64 编译安装成功