R can't find existing header file GL/gl.h during install.packages("rgl") - r

Question

I am having trouble installing 'rgl' on a Linux virtual machine (Ubuntu 18.04 - details at bottom). There are several related questions that are resolved by installing the required dependencies, however in my instance those dependency files exist and are not found by R when installing the R package "rgl".
The files in question are GL/gl.h and GL/glu.h
When running the standard install.packages("rgl"), R gives me this error in the installation report:
checking GL/gl.h usability... no
checking GL/gl.h presence... no
checking for GL/gl.h... no
checking GL/glu.h usability... no
checking GL/glu.h presence... no
checking for GL/glu.h... no
configure: error: missing required header GL/gl.h
ERROR: configuration failed for package ‘rgl’
* removing ‘/opt/conda/lib/R/library/rgl’
Warning in install.packages :
installation of package ‘rgl’ had non-zero exit status
However, those files do exist in /usr/include/GL/ as shown here:
root#[REDACTED]:/usr/include/GL# ls -la
total 1520
drwxr-xr-x 1 root root 6 Oct 27 23:38 .
drwxr-xr-x 1 root root 4096 Oct 27 23:36 ..
-rw-r--r-- 1 root root 80393 Jun 12 01:21 gl.h
-rw-r--r-- 1 root root 421419 Jun 12 01:21 glcorearb.h
-rw-r--r-- 1 root root 848217 Jun 12 01:21 glext.h
-rw-r--r-- 1 root root 17255 May 21 2016 glu.h
-rw-r--r-- 1 root root 3315 May 21 2016 glu_mangle.h
-rw-r--r-- 1 root root 14578 Jun 12 01:21 glx.h
-rw-r--r-- 1 root root 48752 Jun 12 01:21 glxext.h
-rw-r--r-- 1 root root 4706 Apr 3 2018 glxint.h
-rw-r--r-- 1 root root 2086 Apr 3 2018 glxmd.h
-rw-r--r-- 1 root root 81112 Apr 3 2018 glxproto.h
-rw-r--r-- 1 root root 11436 Apr 3 2018 glxtokens.h
drwxr-xr-x 1 root root 37 Sep 22 04:24 internal
I tried installing the package from the terminal using this commands:
wget https://cloud.r-project.org/src/contrib/rgl_0.100.54.tar.gz
then
R CMD INSTALL --configure-args="--with-gl-includes=/usr/include/GL/" rgl_0.100.54.tar.gz
but that didn't work either.
TBH, I am not a Linux expert, so not sure if I'm close and making simple errors, or if there is something major happening under the hood to prevent this from working. I also tried quite a few things with the PATH variable in both R (using Sys.setenv("PATH"="[LONG-PATH-HERE]")) and in the terminal. When looking in the rgl package 'config' file, it seems like it is looking in the right place for the files, but I'm not sure how that file executes.
OS version:
LSB Version: core-9.20170808ubuntu1-noarch:printing-9.20170808ubuntu1-noarch:security-9.20170808ubuntu1-noarch
Distributor ID: Ubuntu
Description: Ubuntu 18.04.5 LTS
Release: 18.04
Codename: bionic
R Version:
> R.version
_
platform x86_64-conda_cos6-linux-gnu
arch x86_64
os linux-gnu
system x86_64, linux-gnu
status
major 3
minor 6.3
year 2020
month 02
day 29
svn rev 77875
language R
version.string R version 3.6.3 (2020-02-29)
nickname Holding the Windsock
Thank you, stack overflow comrades.