unix: Allow to build libffi from source and link against it.

Linking against local libffi (and other libs in future) is triggered by
"make MICROPY_STANDALONE=1". Before that, dependent libs should be built
with "make deplibs".

unix/Makefile

@@ -77,13 +77,26 @@ ifeq ($(MICROPY_PY_SOCKET),1)
 CFLAGS_MOD += -DMICROPY_PY_SOCKET=1
 SRC_MOD += modsocket.c
 endif
+
 ifeq ($(MICROPY_PY_FFI),1)
-LIBFFI_LDFLAGS_MOD := $(shell pkg-config --libs libffi)
+
+ifeq ($(MICROPY_STANDALONE),1)
+LIBFFI_CFLAGS_MOD := -I$(shell ls -1d ../lib/libffi/out/lib/libffi-*/include)
+ ifeq ($(MICROPY_FORCE_32BIT),1)
+  LIBFFI_LDFLAGS_MOD = ../lib/libffi/out/lib32/libffi.a
+ else
+  LIBFFI_LDFLAGS_MOD = ../lib/libffi/out/lib/libffi.a
+ endif
+else
 LIBFFI_CFLAGS_MOD := $(shell pkg-config --cflags libffi)
-CFLAGS_MOD += $(LIBFFI_CFLAGS_MOD) -DMICROPY_PY_FFI=1
+LIBFFI_LDFLAGS_MOD := $(shell pkg-config --libs libffi)
+endif
+
 ifeq ($(UNAME_S),Linux)
-LDFLAGS_MOD += -ldl
+LIBFFI_LDFLAGS_MOD += -ldl
 endif
+
+CFLAGS_MOD += $(LIBFFI_CFLAGS_MOD) -DMICROPY_PY_FFI=1
 LDFLAGS_MOD += $(LIBFFI_LDFLAGS_MOD)
 SRC_MOD += modffi.c
 endif
@@ -172,3 +185,19 @@ $(BUILD)/frozen_upip/upip.py: $(UPIP_TARBALL)
 	$(Q)rm -rf $(dir $@)
 	$(Q)mkdir -p $(dir $@)
 	$(Q)cp $(BUILD)/micropython-upip-*/upip*.py $(dir $@)
+
+
+# Value of configure's --host= option (required for cross-compilation).
+# Deduce it from CROSS_COMPILE by default, but can be overriden.
+ifneq ($(CROSS_COMPILE),)
+CROSS_COMPILE_HOST = --host=$(patsubst %-,%,$(CROSS_COMPILE))
+else
+CROSS_COMPILE_HOST =
+endif
+
+deplibs: libffi
+
+libffi:
+	cd ../lib/libffi; git clean -d -x -f
+	cd ../lib/libffi; ./autogen.sh
+	cd ../lib/libffi; ./configure $(CROSS_COMPILE_HOST) --prefix=$$PWD/out CC="$(CC)" CXX="$(CXX)" LD="$(LD)"; make install