Higher screen resolution Slackware guest in virtualbox.

In the past I used to have perfect screen resolution with a Slackware guest in virtualbox, but for a while now (Slackware 14.2, virtualbox 4.3.24), compiling Guest Additions fails. It ends with:

/tmp/vbox.0/lnkops.c:58:23: note: (near initialization for 'sf_lnk_iops.put_link')
cc1: some warnings being treated as errors
scripts/Makefile.build:258: recipe for target '/tmp/vbox.0/lnkops.o' failed
make[2]: *** [/tmp/vbox.0/lnkops.o] Error 1
Makefile:1384: recipe for target '_module_/tmp/vbox.0' failed
make[1]: *** [_module_/tmp/vbox.0] Error 2
/tmp/vbox.0/Makefile.include.footer:79: recipe for target 'vboxsf' failed
make: *** [vboxsf] Error 2
Creating user for the Guest Additions.
Creating udev rule for the Guest Additions kernel module.

Unsurprisingly, when I boot the guest, modprobe fails as well:

Starting the VirtualBox Guest Additions ...fail!
(modprobe vboxsf failed)
Starting VirtualBox Guest Addition service ...done. 

Therefore I’m stuck on a tiny screen resolution. After almost two hours of searching, I had to conclude that no official solution exists. I did find a less perfect experimental solution:

https://www.whonix.org/wiki/Higher_Screen_Resolution

Following the instructions there gives me a maximum resolution of 1600x1200. Not quite the size of my screen, but I think I can live with it. (If the site is down or page is gone, I saved the relevant bit here).

Notes:

(1) xrandr gives now:

Screen 0: minimum 320 x 200, current 1600 x 1200, maximum 1600 x 1200
default connected 1600x1200+0+0 0mm x 0mm
   1600x1200     86.00* 
   1280x1024     86.00  
   1152x864      76.00  
   1024x768      85.00  
   800x600       86.00  
   640x480       86.00  
   640x400       86.00  
   320x200        0.00  

(2) I had to uninstall the Guest Additions, which turned out to be unintuitive. Google gave a somewhat unhelpful page: Re: How to uninstall Guest Additions in Ubuntu guest? until someone added to it:

Postby TheBigKahoona » 27. May 2011, 13:58
for everyone how ends up here by googleing this matter:
/opt/[VboxAddonsFolder]/uninstall.sh

That was nice.