Configuring with cooling does not support out of source build

At the moment we support out of source builds but not with cooling.

HEAD: b73901b5 (latest master)

Reproduction steps:

git clone git@gitlab.cosma.dur.ac.uk:swift/swiftsim.git
cd swiftsim
./autogen.sh

mkdir build
cd build
../configure --with-cooling=EAGLE --with-chemistry=EAGLE --disable-hand-vec
make -j

>>>swiftsim/build/eagle/examples/Cooling/CoolingRates'
mpicc > ... -MT cooling_rates-cooling_rates.o -MD -MP -MF .deps/cooling_rates-cooling_rates.Tpo -c -o cooling_rates-cooling_rates.o `test -f 'cooling_rates.c' || echo '../../../../../examples/Cooling/CoolingRates/'`cooling_rates.c
../../../../../examples/Cooling/CoolingRates/cooling_rates.c(19): catastrophic error: cannot open source file "../config.h"
  #include "../config.h"
                        ^

compilation aborted for ../../../../../examples/Cooling/CoolingRates/cooling_rates.c (code 4)

This uses the Intel compiler 2019, using the following modules on COSMA:

module purge
module load intel_comp/2019 intel_mpi/2019 fftw parallel_hdf5/1.10.3 gsl python/3.6.5