Troubleshooting Virtualbox shared folders as it relates to Linux headers

Trouableshoot

Host: Windows 7

Guest: Debian Linux

Tried: Installing Guest Additions, installing headers, re-installing headers, installing DKMS, creating vboxsf group/users

Problem: Guest Additions appears to have problems with headers or the keel configuration or something...

Guest Additions has been inserted installed several times, but it keeps failing on the Shared Folders section of the installation.

The problem seems to be with the linux headers, though I've installed, uninstalled, and re-installed what seem to be the correct headers based on uname -a.

root@debian:/media# uname -a
Linux debian 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt20-1+deb8u3 (2016-01-17) x86_64 GNU/Linux
root@debian:/media# apt-get install linux-headers-3.16.0.4
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Note, selecting 'linux-headers-3.16.0-4-all' for regex 'linux-headers-3.16.0.4'
Note, selecting 'linux-headers-3.16.0-4-amd64' for regex 'linux-headers-3.16.0.4'
Note, selecting 'linux-headers-3.16.0-4-common' for regex 'linux-headers-3.16.0.4'
Note, selecting 'linux-headers-3.16.0-4-all-amd64' for regex 'linux-headers-3.16.0.4'
linux-headers-3.16.0-4-amd64 is already the newest version.
linux-headers-3.16.0-4-common is already the newest version.
linux-headers-3.16.0-4-common set to manually installed.

after the successful installation of the headers, I try installing Guest Additions (Shared Folders support):

root@debian:/media# sh /media/cdrom/VBoxLinuxAdditions.run
Verifying archive integrity... All good.
Uncompressing VirtualBox 4.3.12 Guest Additions for Linux............
VirtualBox Guest Additions installer
Removing installed version 4.3.12 of VirtualBox Guest Additions...
Copying additional installer modules ...
Installing additional modules ...
Removing existing VirtualBox DKMS keel modules ...done.
Removing existing VirtualBox non-DKMS keel modules ...done.
Building the VirtualBox Guest Additions keel modules
The headers for the current ruing keel were not found. If the following
module compilation fails then this could be the reason.

Building the main Guest Additions module ...done.
Building the shared folder support module ...fail!
(Look at /var/log/vboxadd-install.log to find out what went wrong)
Doing non-keel setup of the Guest Additions ...done.
Installing the Window System drivers
Waing: unknown version of the X Window System installed.  Not installing
X Window System drivers.
 ...done.
Installing graphics libraries and desktop services components ...done.

So, I had a look at the log file mentioned in the error message above:

Uninstalling modules from DKMS Attempting to install using DKMS

Creating symlink /var/lib/dkms/vboxguest/4.3.12/source ->
                 /usr/src/vboxguest-4.3.12

DKMS: add completed.

Keel preparation uecessary for this keel.  Skipping...

Building module:
cleaning build area....
make KERNELRELEASE=3.16.0-4-amd64 -C /lib/modules/3.16.0-4-amd64/build M=/var/lib/dkms/vboxguest/4.3.12/build............(bad$
Error! Bad retu status for module build on keel: 3.16.0-4-amd64 (x86_64)
Consult /var/lib/dkms/vboxguest/4.3.12/build/make.log for more information.
Failed to install using DKMS, attempting to install without
grep: /lib/modules/3.16.0-4-amd64/build/include/linux/version.h: No such file or directory
make KBUILD_VERBOSE=1 CONFIG_MODULE_SIG= -C /lib/modules/3.16.0-4-amd64/build SUBDIRS=/tmp/vbox.0 SRCROOT=/tmp/vbox.0 modules
Makefile:10: *** mixed implicit and normal rules: deprecated syntax
make[1]: Entering directory `/usr/src/linux-headers-3.16.0-4-amd64'
make -C /usr/src/linux-headers-3.16.0-4-amd64 
KBUILD_SRC=/usr/src/linux-headers-3.16.0-4-common 
KBUILD_EXTMOD="/tmp/vbox.0" -f /usr/src/linux-headers-3.16.0-4-common/Makefile 
modules
test -e include/generated/autoconf.h -a -e include/config/auto.conf || (                
echo >&2;                                                       
echo >&2 "  ERROR: Keel configuration is invalid.";           
echo >&2 "         include/generated/autoconf.h or include/config/auto.conf are missing.";
echo >&2 "         Run 'make oldconfig && make prepare' on keel src to fix it.";      
echo >&2 ;                                                      
/bin/false)
mkdir -p /tmp/vbox.0/.tmp_versions ; rm -f /tmp/vbox.0/.tmp_versions/*
make -f /usr/src/linux-headers-3.16.0-4-common/scripts/Makefile.build obj=/tmp/vbox.0
   gcc-4.8 -Wp,-MD,/tmp/vbox.0/.VBoxGuest-linux.o.d  -nostdinc -isystem /usr/lib/gcc/x86_64-linux-gnu/4.8/include -I/usr/src/$
  if [ "-pg" = "-pg" ]; then if [ /tmp/vbox.0/VBoxGuest-linux.o != "scripts/mod/empty.o" ]; then ./scripts/recordmcount  "/tm$
   gcc-4.8 -Wp,-MD,/tmp/vbox.0/.VBoxGuest.o.d  -nostdinc -isystem /usr/lib/gcc/x86_64-linux-gnu/4.8/include -I/usr/src/linux-$
/tmp/vbox.0/VBoxGuest.c: In function ‘VBoxGuestCommonGetHandledEventsLocked’:
/tmp/vbox.0/VBoxGuest.c:82:5: waing: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
     uint32_t u32AllowedGuestCaps = pSession->u32AquiredGuestCaps | (VMMDEV_EVENT_VALID_EVENT_MASK & ~pDevExt->u32AcquireMode$

I searched on one of the early errors in the log file and found this. Not sure if it's related:

https://github.com/zfsonlinux/zfs/issues/3807