add C++ compiler settings to octopus configure wrappers

octopus/foss2021a-cuda-mpi-config.sh

 export CC="mpicc"
 export CFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments"
+export CXX="mpicxx"
+export CXXFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments"
 export FC="mpif90"
 export FCFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments -ffree-line-length-none -fallow-argument-mismatch -fallow-invalid-boz"
 

octopus/foss2021a-mpi-config.sh

 export CC="mpicc"
 export CFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments"
+export CXX="mpicxx"
+export CXXFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments"
 export FC="mpif90"
 export FCFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments -ffree-line-length-none -fallow-argument-mismatch -fallow-invalid-boz"
 

octopus/foss2021a-serial-config.sh

 export CC="gcc"
 export CFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments"
+export CXX="g++"
+export CXXFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments"
 export FC="gfortran"
 export FCFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments -ffree-line-length-none -fallow-argument-mismatch -fallow-invalid-boz"
 

octopus/foss2022a-cuda-mpi-config.sh

 export CC="mpicc"
 export CFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments"
+export CXX="mpicxx"
+export CXXFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments"
 export FC="mpif90"
 export FCFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments -ffree-line-length-none -fallow-argument-mismatch -fallow-invalid-boz"
 

octopus/foss2022a-mpi-config.sh

 export CC="mpicc"
 export CFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments"
+export CXX="mpicxx"
+export CXXFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments"
 export FC="mpif90"
 export FCFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments -ffree-line-length-none -fallow-argument-mismatch -fallow-invalid-boz"
 

octopus/foss2022a-serial-config.sh

 export CC="gcc"
 export CFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments"
+export CXX="g++"
+export CXXFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments"
 export FC="gfortran"
 export FCFLAGS="${GCC_ARCH:+-march=$GCC_ARCH }-O2 -g -fno-var-tracking-assignments -ffree-line-length-none -fallow-argument-mismatch -fallow-invalid-boz"