Linux – How to get the clipboard to work in CentOS inside VirtualBox on Windows

I have CentOS 7.3.1611 running with XFCE inside Oracle VirtualBox 5.1.16 on Windows 8.1. The other questions are for older versions or the guest/host are switched. I set Devices > Shared Clipboard > Bidirectional, but nothing I copy in Windows is pasteable in CentOS, and nothing I copy in CentOS is pasteable in Windows.

I used Vagrant to create the box. vagrant init centos/7; vagrant up

==> default: Checking for guest additions in VM...
    default: No guest additions were detected on the base box for this VM! Guest
    default: additions are required for forwarded ports, shared folders, host only
    default: networking, and more. If SSH fails on this machine, please install
    default: the guest additions and repackage the box to continue.
    default:
    default: This is not an error message; everything may continue to work properly,

I also manually added an optical drive and loaded VBoxGuestAdditions.iso from the VirtualBox folder, restarted, manually mount the CDROM, and manually ran the script, but:

[vagrant@localhost ~]$ sudo mount /dev/sr0/ /mnt
mount: /dev/sr0 is write-protected, mounting read-only
[vagrant@localhost ~]$ cd /mnt
[vagrant@localhost mnt]$ sudo ./autorun.sh
Linux guest additions installer not found -- try to start them manually.
[vagrant@localhost mnt]$ ls
32Bit  AUTORUN.INF  cert  runasroot.sh            VBoxSolarisAdditions.pkg        VBoxWindowsAdditions.exe
64Bit  autorun.sh   OS2   VBoxLinuxAdditions.run  VBoxWindowsAdditions-amd64.exe  VBoxWindowsAdditions-x86.exe

So now I don't know how to get the clipboard to work.

Here is the log from running VBoxLinuxAdditions.run.

[vagrant@localhost mnt]$ sudo ./VBoxLinuxAdditions.run
Verifying archive integrity... All good.
Uncompressing VirtualBox 5.1.16 Guest Additions for Linux...........
VirtualBox Guest Additions installer
Removing installed version 5.1.16 of VirtualBox Guest Additions...
Copying additional installer modules ...
Installing additional modules ...
vboxadd.sh: Building Guest Additions kernel modules.
Failed to set up service vboxadd, please check the log file
/var/log/VBoxGuestAdditions.log for details.
[vagrant@localhost mnt]$ cat /var/log/VBoxGuestAdditions.log

vboxadd.sh: failed: Look at /var/log/vboxadd-install.log to find out what went wrong.
vboxadd.sh: failed: Please check that you have gcc, make, the header files for your Linux kernel and possibly perl installed..
[vagrant@localhost mnt]$ cat /var/log/vboxadd-install.log
/tmp/vbox.0/Makefile.include.header:97: *** Error: unable to find the sources of your current Linux kernel. Specify KERN_DIR=<directory> and run Make again.  Stop.
Creating user for the Guest Additions.
Creating udev rule for the Guest Additions kernel module.
[vagrant@localhost mnt]$ gcc
gcc: fatal error: no input files
compilation terminated.
[vagrant@localhost mnt]$