Fix vel

Added config option to compile VELOCIraptor interface in SWIFT without having the actual VELOCIraptor library.

src/Makefile.am

@@ -61,7 +61,7 @@ AM_SOURCES = space.c runner.c queue.c task.c cell.c engine.c \
     part_type.c xmf.c gravity_properties.c gravity.c \
     collectgroup.c hydro_space.c equation_of_state.c \
     chemistry.c cosmology.c restart.c mesh_gravity.c velociraptor_interface.c \
-    outputlist.c
+    outputlist.c velociraptor_dummy.c
 
 # Include files for distribution, not installation.
 nobase_noinst_HEADERS = align.h approx_math.h atomic.h barrier.h cycle.h error.h inline.h kernel_hydro.h kernel_gravity.h \