Error in running the "make" command in installing RegCM

I am trying to install RegCM-4.7.1 on Ubuntu but I keep getting this error (screenshot attached) in an attempt to run the "make" command. Please help me. I am using "GNU Fortran (Ubuntu 10.3.0-1ubuntu1) 10.3.0" and I have tried installing packages such as binutils, zlib, hdf5, netcdf, etc .... thanks in advance. enter image description here

the output of the "make" command is as follows:

make  all-recursive
make[1]: Entering directory '/home/josh/Downloads/RegCM-4.7.1'
Making all in external
make[2]: Entering directory '/home/josh/Downloads/RegCM-4.7.1/external'
make[2]: Nothing to be done for 'all'.
make[2]: Leaving directory '/home/josh/Downloads/RegCM-4.7.1/external'
Making all in Share
make[2]: Entering directory '/home/josh/Downloads/RegCM-4.7.1/Share'
gfortran -I/home/josh/Downloads/RegCM-4.7.1/external -I/usr/include -I/usr/include/hdf5/serial -I/usr/include -I/usr/include -I/usr/include -I/usr/include/hdf5/serial -I/usr/include -I/usr/include  -I/home/josh/Downloads/RegCM-4.7.1/Share -fopenmp -D'SVN_REV="tag-4.7.1"' -O3 -finline-functions -mtune=native -march=native -fconvert=big-endian -fno-range-check -fPIC -c mod_realkinds.F90
mod_realkinds.F90:63:31:

   63 |   real(rk8), parameter :: inf = O'0777600000000000000000'
      |                               1
Error: BOZ literal constant at (1) is neither a data-stmt-constant nor an actual argument to INT, REAL, DBLE, or CMPLX intrinsic function [see ‘-fno-allow-invalid-boz’]
mod_realkinds.F90:64:31:

   64 |   real(rk8), parameter :: nan = O'0777610000000000000000'
      |                               1
Error: BOZ literal constant at (1) is neither a data-stmt-constant nor an actual argument to INT, REAL, DBLE, or CMPLX intrinsic function [see ‘-fno-allow-invalid-boz’]
make[2]: *** [Makefile:613: mod_realkinds.o] Error 1
make[2]: Leaving directory '/home/josh/Downloads/RegCM-4.7.1/Share'
make[1]: *** [Makefile:433: all-recursive] Error 1
make[1]: Leaving directory '/home/josh/Downloads/RegCM-4.7.1'
make: *** [Makefile:365: all] Error 2

Ubuntu 21.04, RegCM4

RegCM can be built with {gcc-9, g++-9, gfortran-9}.

sudo apt install gcc-9 g++-9 gfortran-9 libnetcdff-dev libnetcdf-dev libopenmpi-dev

cd RegCM-4.7.1/
bash bootstrap.sh
export CC=gcc-9 CXX=g++-9 FC=gfortran-9
./configure
 make                        ## no errors