external: make submodule hacks more general, and common.

We can't rely on the existence or not of a header, as Carl's
delete-and-make-submodule change showed.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

external/Makefile

@@ -20,20 +20,20 @@ EXTERNAL_INCLUDE_FLAGS :=					\
 
 EXTERNAL_LDLIBS := -Lexternal $(patsubst lib%.a,-l%,$(notdir $(EXTERNAL_LIBS)))
 
+# Might exist, but need updating.  Nuke and rebuild.
+submodcheck-%: FORCE
+	if git submodule status external/$* | grep -q '^[-+]'; then rm -rf external/$*; git submodule update --init external/$*; fi
+
 # We build libsodium, since Ubuntu xenial has one too old.
 external/libsodium.a: external/libsodium/src/libsodium/libsodium.la
 	$(MAKE) -C external/libsodium install-exec
 
-external/libsodium/src/libsodium/include/sodium.h:
-	git submodule update external/libsodium
-	[ -f $@ ] || git submodule update --init external/libsodium
+external/libsodium/src/libsodium/include/sodium.h: submodcheck-libsodium
 
 external/libsodium/src/libsodium/libsodium.la: external/libsodium/src/libsodium/include/sodium.h
 	cd external/libsodium && ./autogen.sh && ./configure CC="$(CC)" --enable-static=yes --enable-shared=no --enable-tests=no --libdir=`pwd`/.. && $(MAKE)
 
-$(LIBWALLY_HEADERS) $(LIBSECP_HEADERS):
-	git submodule update external/libwally-core
-	[ -f $@ ] || git submodule update --init external/libwally-core
+$(LIBWALLY_HEADERS) $(LIBSECP_HEADERS): submodcheck-libwally-core
 
 # libsecp included in libwally.
 # Wildcards here are magic.  See http://stackoverflow.com/questions/2973445/gnu-makefile-rule-generating-a-few-targets-from-a-single-source-file
@@ -43,10 +43,7 @@ external/libsecp256k1.% external/libwallycore.%: external/libwally-core/src/secp
 external/libwally-core/src/libwallycore.% external/libwally-core/src/secp256k1/libsecp256k1.%: $(LIBWALLY_HEADERS) $(LIBSECP_HEADERS)
 	cd external/libwally-core && ./tools/autogen.sh && ./configure CC="$(CC)" --enable-static=yes --enable-module-recovery --enable-shared=no --libdir=`pwd`/.. && $(MAKE)
 
-# Git submodules are seriously broken.
-external/jsmn/jsmn.h:
-	git submodule update external/jsmn/
-	[ -f $@ ] || git submodule update --init external/jsmn/
+external/jsmn/jsmn.h: submodcheck-jsmn
 
 # If we tell Make that the above builds both, it runs it twice in
 # parallel.  So we lie :(
@@ -63,9 +60,7 @@ LIBBASE58_SRC := external/libbase58/base58.c
 
 $(LIBBASE58_SRC): $(LIBBASE58_HEADERS)
 
-$(LIBBASE58_HEADERS):
-	git submodule update external/libbase58/ || true
-	[ -f $@ ] || git submodule update --init external/libbase58/
+$(LIBBASE58_HEADERS): submodcheck-libbase58
 
 # Can't be inside submodule, as that makes git think it's dirty.
 external/base58.o: $(LIBBASE58_SRC) Makefile
@@ -74,9 +69,7 @@ external/base58.o: $(LIBBASE58_SRC) Makefile
 external/libbase58.a: external/base58.o
 	$(AR) rc $@ $<
 
-external/libbacktrace/backtrace.h:
-	git submodule update external/libbacktrace/
-	[ -f $@ ] || git submodule update --init external/libbacktrace/
+external/libbacktrace/backtrace.h: submodcheck-libbacktrace
 
 # Need separate build dir: changes inside submodule make git think it's dirty.
 external/libbacktrace.a: external/libbacktrace/backtrace.h