unix: Build libffi in a directory which is gitgnored.

To avoid "-dirty" version previous and spurious "modified" output from
git status, etc.

unix/Makefile

@@ -81,11 +81,11 @@ endif
 ifeq ($(MICROPY_PY_FFI),1)
 
 ifeq ($(MICROPY_STANDALONE),1)
-LIBFFI_CFLAGS_MOD := -I$(shell ls -1d ../lib/libffi/out/lib/libffi-*/include)
+LIBFFI_CFLAGS_MOD := -I$(shell ls -1d ../lib/libffi/build_dir/out/lib/libffi-*/include)
  ifeq ($(MICROPY_FORCE_32BIT),1)
-  LIBFFI_LDFLAGS_MOD = ../lib/libffi/out/lib32/libffi.a
+  LIBFFI_LDFLAGS_MOD = ../lib/libffi/build_dir/out/lib32/libffi.a
  else
-  LIBFFI_LDFLAGS_MOD = ../lib/libffi/out/lib/libffi.a
+  LIBFFI_LDFLAGS_MOD = ../lib/libffi/build_dir/out/lib/libffi.a
  endif
 else
 LIBFFI_CFLAGS_MOD := $(shell pkg-config --cflags libffi)
@@ -205,7 +205,8 @@ deplibs: libffi axtls
 libffi:
 	cd ../lib/libffi; git clean -d -x -f
 	cd ../lib/libffi; ./autogen.sh
-	cd ../lib/libffi; ./configure $(CROSS_COMPILE_HOST) --prefix=$$PWD/out CC="$(CC)" CXX="$(CXX)" LD="$(LD)"; make install
+	mkdir -p ../lib/libffi/build_dir; cd ../lib/libffi/build_dir; \
+	../configure $(CROSS_COMPILE_HOST) --prefix=$$PWD/out CC="$(CC)" CXX="$(CXX)" LD="$(LD)"; make install
 
 axtls:
 	cd ../lib/axtls; cp config/upyconfig config/.config