Compiling with `clang`

When trying to compile under clang with a default clone of master, I get the following warning in the gravity:

libtool: compile:  gcc -DHAVE_CONFIG_H -I. -I.. -I/usr/local/opt/szip/include -I/usr/local/Cellar/hdf5/1.10.5_1/include -O3 -fomit-frame-pointer -fstrict-aliasing -ffast-math -funroll-loops -march=skylake -mavx2 -pthread -Wall -Wextra -Wno-unused-parameter -Wshadow -Werror -Wstrict-prototypes -MT libswiftsim_la-runner_doiact_grav.lo -MD -MP -MF .deps/libswiftsim_la-runner_doiact_grav.Tpo -c runner_doiact_grav.c  -fno-common -DPIC -o .libs/libswiftsim_la-runner_doiact_grav.o
runner_doiact_grav.c:1413:20: error: function 'runner_dopair_recursive_grav_pm' is not needed and will not be emitted
      [-Werror,-Wunneeded-internal-declaration]
static INLINE void runner_dopair_recursive_grav_pm(struct runner *r,
                   ^
1 error generated.