Avoid building runtime/gen/basis-ffi.{h,sml} with clean sources

6a466d9 was a previous attempt to avoid building
`runtime/gen/basis-ffi.{h,sml}` when building from clean sources.
This avoids the need for a working `mlton` when building the runtime,
which is convenient for cross-compiling and porting.

It typically worked, because a fresh `git clone` or `tar x` would
create the `runtime/gen/basis-ffi.{h,sml}` and
`runtime/gen/gen-basis-ffi.{sml,stamp}` files with sufficiently close
timestamps that `make` wouldn't consider anything to be out of date.

However, this is not guaranteed.

This commit removes `runtime/gen/gen-basis-ffi.stamp` and adds
`runtime/gen/basis-ffi.{h,sml}.chk` files, which are SHA checksums of
the inputs and outputs of `runtime/gen/basis-ffi.{h,sml}`.  If the
checksum of the current sources matches the saved checksum, then the
targets can be brought up to date with a simple `touch`.  If an input
has changed, then the targets (and the checksum) are updated by
building and running the `runtime/gen/gen-basis-ffi` tool.

runtime/Makefile

@@ -160,10 +160,23 @@ gen/gen-types$(EXE): util.o
 basis-ffi.h: gen/basis-ffi.h
 	cp $< $@
 
+ifeq ($(shell cat gen/gen-basis-ffi.sml gen/basis-ffi.def gen/basis-ffi.h | shasum -c -s gen/basis-ffi.h.chk && echo true),true)
+gen/basis-ffi.h: gen/gen-basis-ffi.sml gen/basis-ffi.def
+	touch gen/basis-ffi.h
+else
 gen/basis-ffi.h: gen/gen-basis-ffi$(EXE) gen/basis-ffi.def
 	./gen/gen-basis-ffi$(EXE) basis-ffi.h < gen/basis-ffi.def > gen/basis-ffi.h
+	cat gen/gen-basis-ffi.sml gen/basis-ffi.def gen/basis-ffi.h | shasum > gen/basis-ffi.h.chk
+endif
+
+ifeq ($(shell cat gen/gen-basis-ffi.sml gen/basis-ffi.def gen/basis-ffi.sml | shasum -c -s gen/basis-ffi.sml.chk && echo true),true)
+gen/basis-ffi.sml: gen/gen-basis-ffi.sml gen/basis-ffi.def
+	touch gen/basis-ffi.sml
+else
 gen/basis-ffi.sml: gen/gen-basis-ffi$(EXE) gen/basis-ffi.def
 	./gen/gen-basis-ffi$(EXE) basis-ffi.sml < gen/basis-ffi.def > gen/basis-ffi.sml
+	cat gen/gen-basis-ffi.sml gen/basis-ffi.def gen/basis-ffi.sml | shasum > gen/basis-ffi.sml.chk
+endif
 
 gen/gen-basis-ffi$(EXE): gen/gen-basis-ffi.sml
 	$(RUN_MLTON) -output gen/gen-basis-ffi$(EXE) gen/gen-basis-ffi.sml

runtime/gen/basis-ffi.h.chk

@@ -0,0 +1 @@
+4a233057e4e638a47178eea4fcc111399e9c81a4  -

runtime/gen/basis-ffi.sml.chk

@@ -0,0 +1 @@
+b17c34fc7d34d4a6b96f29017ba52173b1b1af65  -