Compiling VirtualBox but need 32-bit headers and libraries

I decided to try to build VirtualBox, but I've discovered that it has a special requirement for 32-bit support on my 64-bit Slackware pre-15 setup:

Code:$ ./configure --with-qt-dir=/usr/local/Qt-5.14.1/
...
Checking for 32-bit support:
Cannot compile 32-bit applications (missing headers and/or libraries)!
Check the file /home/me/Downloads/VirtualBox-6.1.2/configure.log for detailed error information.
Check /home/me/Downloads/VirtualBox-6.1.2/configure.log for detailsDoes anywhere know where I can get these headers and libraries?

Here is the end of the configure log:
Code:***** Checking 32-bit support *****
compiling the following source file:
#include <stdint.h>
int main(void)
{
return 0;
}
using the following command line:
gcc -m32 -O -Wall -o /home/me/Downloads/VirtualBox-6.1.2/.tmp_out /home/me/Downloads/VirtualBox-6.1.2/.tmp_src.c
In file included from /usr/include/features.h:474,
from /usr/include/bits/libc-header-start.h:33,
from /usr/include/stdint.h:26,
from /usr/lib64/gcc/x86_64-slackware-linux/9.2.0/include/stdint.h:9,
from /home/me/Downloads/VirtualBox-6.1.2/.tmp_src.c:1:
/usr/include/gnu/stubs.h:7:11: fatal error: gnu/stubs-32.h: No such file or directory
7 | # include <gnu/stubs-32.h>
| ^~~~~~~~~~~~~~~~
compilation terminated.