Makefile: fix parallel `update-mocks` build.

We always rebuild headerversions to examine critical system headers,
however that stomps on parallel builds with:

make[1]: execvp: tools/headerversions: Text file busy

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

Makefile

@@ -385,8 +385,9 @@ gen_version.h: FORCE
 	@if cmp $@.new $@ >/dev/null 2>&2; then rm -f $@.new; else mv $@.new $@; echo Version updated; fi
 
 # We force make to relink this every time, to detect version changes.
+# Do it atomically, otherwise parallel builds can get upset!
 tools/headerversions: FORCE tools/headerversions.o $(CCAN_OBJS)
-	@$(LINK.o) tools/headerversions.o $(CCAN_OBJS) $(LOADLIBES) $(LDLIBS) -o $@
+	@trap "rm -f $@.tmp.$$$$" EXIT; $(LINK.o) tools/headerversions.o $(CCAN_OBJS) $(LOADLIBES) $(LDLIBS) -o $@.tmp.$$$$ && mv $@.tmp.$$$$ $@
 
 # That forces this rule to be run every time, too.
 gen_header_versions.h: tools/headerversions