gsl linking error when using gcc and disabled mpi

Hi Peter,

It would be great if you can help to take a look at the following issue on cosma7:

## Summary

The code (swift master branch) fails to link gsl library if we use gcc (see the following modules) and disable mpi.


## Steps to reproduce
On cosma7:
`
module purge
module load gnu_comp/7.3.0
module load intel_mpi/2018
module load gsl/2.4
module load hdf5/1.8.20
module load parmetis/4.0.3
module load fftw/3.3.7
`

then

`./configure --disable-mpi
make`


## Error msg:

../src/.libs/libswiftsim.a(libswiftsim_la-neutrino_response.o): In function `neutrino_response_init':
neutrino_response.c:(.text+0x17ac): undefined reference to `gsl_spline2d_free'