ccanlint: license_depends_compat checks dependencies are compatible.

We don't check external dependencies, but internal ccan deps are
pretty easy.

tools/ccanlint/tests/license_depends_compat.c

+#include <tools/ccanlint/ccanlint.h>
+#include <ccan/foreach/foreach.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+#include <limits.h>
+#include <errno.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <err.h>
+#include <ccan/talloc/talloc.h>
+#include <ccan/str/str.h>
+#include <ccan/str_talloc/str_talloc.h>
+
+static void check_license_depends_compat(struct manifest *m,
+					 bool keep,
+					 unsigned int *timeleft,
+					 struct score *score)
+{
+	struct manifest *i;
+
+	score->pass = true;
+
+	/* If our license is unknown, we can't know the answer. */
+	if (m->license == LICENSE_UNKNOWN) {
+		score->score = score->total = 0;
+		return;
+	}
+
+	list_for_each(&m->deps, i, list) {
+		struct doc_section *d = find_license_tag(i);
+		i->license = which_license(d);
+
+		if (i->license != LICENSE_UNKNOWN
+		    && !license_compatible[m->license][i->license]) {
+			score_file_error(score, i->info_file, 0,
+					 "Dependency ccan/%s has"
+					 " incompatible license '%s'",
+					 i->basename,
+					 licenses[i->license].name);
+			score->pass = false;
+		}
+	}
+
+	if (score->pass)
+		score->score = score->total;
+}
+
+struct ccanlint license_depends_compat = {
+	.key = "license_depends_compat",
+	.name = "CCAN dependencies don't contain incompatible licenses",
+	.check = check_license_depends_compat,
+	.needs = "license_exists depends_exist"
+};
+REGISTER_TEST(license_depends_compat);