Linking legacy Fortran95 library to C++ with ifort/icpc - c++

Question

Background: I am in a situation where I have to make use of an old Fortran95 library in a new C++ project. The F95 library is extensive, has tons of small modules, poorly documented, and it was mostly auto-generated about a decade ago by some obscure computer algebra system (by people on a different continent). So basically heirloom code, but it works and it is currently irreplaceable.
Luckily I have the source code and it can be compiled with current versions of ifort, but I am not too familiar with Fortran, and would rather not touch the old code in any significant way.
So suppose I have this Fortran code (pes_shell.f90):
subroutine pes_init()
use pes,wp=>pes_wp
implicit none
real,parameter::auang=0.5291772083
call pes0_init (dir='coef')
call pes1_init (pes_x3y1z1u1_sysall)
return
end subroutine pes_init
The functions pes0_init(...) and pes1_init(...) lead into the abyssal depths of the Fortran library and they are contained in the pes module.
I can compile this to an object file, if I give ifort the path for the modules:
ifort -r8 -O2 -c pes_shell.f90 -I/home/debianuser/PES/PES_library/lib/mod
My POC C++ code, calling pes_init():
extern "C"{
void pes_init_();
}
int main(){
pes_init_();
return 0;
}
This can also be compiled to an object file, with icpc:
icpc -c PEStest.cpp
However I cannot figure out how to link the two object files, plus the truckload of fortran modules, into a final executable.
I have tried just simply using icpc, but it cannot seem to be able to find the Fortran functions, even if I give it the location of the module files:
icpc -I/home/debianuser/PES/PES_library/lib/mod -o test.x pes_shell.o PEStest.o
pes_shell.o: In function `pes_shell_mp_f_':
pes_shell.f90:(.text+0x595): undefined reference to `pes_x3y1z1u1_mp_pes_x3y1z1u1_pot_'
pes_shell.o: In function `pes_shell_mp_pes_init_':
pes_shell.f90:(.text+0x5f0): undefined reference to `pes0_mp_pes0_init_'
pes_shell.f90:(.text+0x603): undefined reference to `pes1c_mp_pes1_init_'
PEStest.o: In function `main':
PEStest.cpp:(.text+0x2b): undefined reference to `pes_init_'
EDIT:
Pointing the linker to the directory where libpes.a can be found eliminates the problem of locating the function that was referenced in the c++ code, but icpc still cannot find the fortran functions that are being called from fortran codes:
icpc -I/home/debianuser/PES/PES_library/lib/mod -L/home/debianuser/PES/PES_library/lib/pes-xyz -lpes -o test.x PEStest.o pes_shell_new.o
pes_shell_new.o: In function `f_':
pes_shell_new.f90:(.text+0x585): undefined reference to `pes_x3y1z1u1_mp_pes_x3y1z1u1_pot_'
pes_shell_new.o: In function `pes_init_':
pes_shell_new.f90:(.text+0x5e0): undefined reference to `pes0_mp_pes0_init_'
pes_shell_new.f90:(.text+0x5f3): undefined reference to `pes1c_mp_pes1_init_'