unix/Makefile: Only checkout libffi submodule when used.

This moves the libffi submodule variable modifier inside of the if
statement where it is actually used so that the submodule will only be
checked out if it is actually being used.

A new DEPLIBS variable is also introduced to prevent building the libffi
submodule when not needed.

Signed-off-by: David Lechner <david@pybricks.com>

ports/unix/Makefile

@@ -30,7 +30,7 @@ UNAME_S := $(shell uname -s)
 include $(TOP)/py/py.mk
 include $(TOP)/extmod/extmod.mk
 
-GIT_SUBMODULES += lib/berkeley-db-1.xx lib/libffi
+GIT_SUBMODULES += lib/berkeley-db-1.xx
 
 INC +=  -I.
 INC +=  -I$(TOP)
@@ -202,13 +202,17 @@ endif
 ifeq ($(MICROPY_PY_FFI),1)
 
 ifeq ($(MICROPY_STANDALONE),1)
+# Build libffi from source.
+GIT_SUBMODULES += lib/libffi
+DEPLIBS += libffi
 LIBFFI_CFLAGS_MOD := -I$(shell ls -1d $(BUILD)/lib/libffi/out/lib/libffi-*/include)
  ifeq ($(MICROPY_FORCE_32BIT),1)
   LIBFFI_LDFLAGS_MOD = $(BUILD)/lib/libffi/out/lib32/libffi.a
  else
   LIBFFI_LDFLAGS_MOD = $(BUILD)/lib/libffi/out/lib/libffi.a
  endif
 else
+# Use system version of libffi.
 LIBFFI_CFLAGS_MOD := $(shell pkg-config --cflags libffi)
 LIBFFI_LDFLAGS_MOD := $(shell pkg-config --libs libffi)
 endif
@@ -325,7 +329,7 @@ else
 CROSS_COMPILE_HOST =
 endif
 
-deplibs: libffi
+deplibs: $(DEPLIBS)
 
 libffi: $(BUILD)/lib/libffi/include/ffi.h