lightningd: fix up installs in subdirectories.

Commit a1fdeee "Makefile: clean up install path handling."
broke the ability to configure with one path and then run in a
different path.  Turns out people actually do this!  So, we have
to use relative paths, compared to our existing binary.

Fixes: ElementsProject#7595
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Makefile

@@ -252,7 +252,7 @@ CPATH := /usr/local/include
 LIBRARY_PATH := /usr/local/lib
 endif
 
-CPPFLAGS += -DCLN_NEXT_VERSION="\"$(CLN_NEXT_VERSION)\"" -DPKGLIBEXECDIR="\"$(pkglibexecdir)\"" -DPLUGINDIR="\"$(plugindir)\"" -DCCAN_TAL_NEVER_RETURN_NULL=1
+CPPFLAGS += -DCLN_NEXT_VERSION="\"$(CLN_NEXT_VERSION)\"" -DPKGLIBEXECDIR="\"$(pkglibexecdir)\"" -DBINDIR="\"$(bindir)\"" -DPLUGINDIR="\"$(plugindir)\"" -DCCAN_TAL_NEVER_RETURN_NULL=1
 CFLAGS = $(CPPFLAGS) $(CWARNFLAGS) $(CDEBUGFLAGS) $(COPTFLAGS) -I $(CCANDIR) $(EXTERNAL_INCLUDE_FLAGS) -I . -I$(CPATH) $(SQLITE3_CFLAGS) $(POSTGRES_INCLUDE) $(FEATURES) $(COVFLAGS) $(DEV_CFLAGS) -DSHACHAIN_BITS=48 -DJSMN_PARENT_LINKS $(PIE_CFLAGS) $(COMPAT_CFLAGS) $(CSANFLAGS)
 
 # If CFLAGS is already set in the environment of make (to whatever value, it
@@ -886,7 +886,7 @@ uninstall:
 installcheck: all-programs
 	@rm -rf testinstall || true
 	$(MAKE) DESTDIR=$$(pwd)/testinstall install
-	DEV_LIGHTNINGD_DESTDIR_PREFIX=$$(pwd)/testinstall/ testinstall$(bindir)/lightningd --test-daemons-only --lightning-dir=testinstall
+	testinstall$(bindir)/lightningd --test-daemons-only --lightning-dir=testinstall
 	$(MAKE) DESTDIR=$$(pwd)/testinstall uninstall
 	@if test `find testinstall '!' -type d | wc -l` -ne 0; then \
 		echo 'make uninstall left some files in testinstall directory!'; \

lightningd/lightningd.c

@@ -534,7 +534,7 @@ static const char *find_my_directory(const tal_t *ctx, const char *argv0)
 static void find_subdaemons_and_plugins(struct lightningd *ld, const char *argv0)
 {
 	const char *my_path = find_my_directory(tmpctx, argv0);
-	const char *prefix;
+	const char *rel;
 
 	/* If we're running in-tree, all the subdaemons are with lightningd. */
 	if (has_all_subdaemons(my_path)) {
@@ -547,14 +547,15 @@ static void find_subdaemons_and_plugins(struct lightningd *ld, const char *argv0
 		return;
 	}
 
-	/* Assume we're running the installed version.  Override
-	 * for "make installccheck" though. */
-	prefix = getenv("DEV_LIGHTNINGD_DESTDIR_PREFIX");
-	if (!prefix)
-		prefix = "";
-	ld->subdaemon_dir = tal_fmt(ld, "%s%s", prefix, PKGLIBEXECDIR);
+	/* OK, we're running the installed version?  But we used to allow
+	 * running in an arbitrary subtree, and people still do (plus,
+	 * installcheck does this).  So we use relative paths here. */
+	rel = path_rel(NULL, BINDIR, PKGLIBEXECDIR);
+	ld->subdaemon_dir = path_join(ld, my_path, take(rel));
+
+	rel = path_rel(NULL, BINDIR, PLUGINDIR);
 	plugins_set_builtin_plugins_dir(ld->plugins,
-					tal_fmt(tmpctx, "%s%s", prefix, PLUGINDIR));
+					path_join(tmpctx, my_path, take(rel)));
 }
 
 /*~ We like to free everything on exit, so valgrind doesn't complain (valgrind