Makefile: don't make every file depend on version headers.

common/version.o depends on common/version_gen.h explicitly already,
and header_versions_gen.h is only used by lightningd, so we can make
that dependency explicit.

This means when version changes (i.e. different git commit) we only
relink, not recompile.

Before:
```
real	0m6.578s
user	0m14.705s
sys	0m13.621s
```

After:
```
real	0m2.098s
user	0m6.763s
sys	0m4.844s
```

Signed-off-by: Rusty Russell <[email protected]>

Makefile

@@ -232,7 +232,7 @@ ALL_TEST_PROGRAMS :=
 ALL_TEST_GEN :=
 ALL_FUZZ_TARGETS :=
 ALL_C_SOURCES :=
-ALL_C_HEADERS := header_versions_gen.h version_gen.h
+ALL_C_HEADERS :=
 # Extra (non C) targets that should be built by default.
 DEFAULT_TARGETS :=
 
@@ -640,7 +640,7 @@ version_gen.h: $(FORCE)
 endif
 
 # That forces this rule to be run every time, too.
-header_versions_gen.h: tools/headerversions
+header_versions_gen.h: tools/headerversions $(FORCE)
 	@tools/headerversions $@
 
 # We make a static library, this way linker can discard unused parts.

lightningd/Makefile

@@ -69,6 +69,9 @@ ALL_C_SOURCES += $(LIGHTNINGD_SRC) $(LIGHTNINGD_SRC_NOHDR)
 ALL_C_HEADERS += $(LIGHTNINGD_HDRS)
 ALL_PROGRAMS += lightningd/lightningd
 
+# We explicitly check header versions in lightningd.c
+lightningd/lightningd.o: header_versions_gen.h
+
 # Common source we use.
 LIGHTNINGD_COMMON_OBJS :=			\
 	common/addr.o				\