adding back LIBSUNDIALS variable with a note to remove it in the future

Author: syclik

make/libraries

@@ -81,6 +81,9 @@ $(SUNDIALS)/lib/libsundials_nvecserial.a: $(SUNDIALS_NVECSERIAL)
 
 SUNDIALS_TARGETS ?= $(SUNDIALS)/lib/libsundials_nvecserial.a $(SUNDIALS)/lib/libsundials_cvodes.a $(SUNDIALS)/lib/libsundials_idas.a $(SUNDIALS)/lib/libsundials_kinsol.a
 
+# FIXME(dl, 02/2023): remove LIBSUNDIALS variable after replacing use in Stan and CmdStan with SUNDIALS_TARGETS
+LIBSUNDIALS = $(SUNDIALS_TARGETS)
+
 STAN_SUNDIALS_HEADERS :=  $(call findfiles,$(MATH)stan,*cvodes*.hpp) $(call findfiles,$(MATH)stan,*idas*.hpp) $(call findfiles,$(MATH)stan,*kinsol*.hpp)
 $(STAN_SUNDIALS_HEADERS) : $(SUNDIALS_TARGETS)