ccanlint: check for #ifdef

Old habits die hard; it's better to use #if <FEATURE> than #ifdef <FEATURE>;
they're similar, because undefined identifiers evaluate to zero, but with
GCC's -Wundef flag you can detect mis-spelled or missing features with
#if.

autoconf-style config.h leave unset features undefined, so this works for
those config.h too.

tools/ccanlint/tests/hash_if.c

+#include <tools/ccanlint/ccanlint.h>
+#include <tools/tools.h>
+#include <ccan/talloc/talloc.h>
+#include <ccan/str/str.h>
+#include <ccan/str_talloc/str_talloc.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 <string.h>
+#include <ctype.h>
+
+static void check_hash_if(struct manifest *m,
+				   bool keep,
+				   unsigned int *timeleft, struct score *score)
+{
+	struct list_head *list;
+	const char *explanation =
+	"\n\t(#if works like #ifdef, but with gcc's -Wundef, we can detect\n"
+	"\tmistyped or unknown configuration options)";
+
+	foreach_ptr(list, &m->c_files, &m->h_files,
+		    &m->run_tests, &m->api_tests,
+		    &m->compile_ok_tests, &m->compile_fail_tests,
+		    &m->other_test_c_files) {
+		struct ccan_file *f;
+
+		list_for_each(list, f, list) {
+			unsigned int i;
+			char **lines = get_ccan_file_lines(f);
+
+			for (i = 0; lines[i]; i++) {
+				const char *line = lines[i];
+				char *sym;
+
+				if (!get_token(&line, "#"))
+					continue;
+				if (!(get_token(&line, "if")
+				      && get_token(&line, "defined")
+				      && get_token(&line, "("))
+				    && !get_token(&line, "ifdef"))
+					continue;
+
+				sym = get_symbol_token(lines, &line);
+				if (!sym || !strstarts(sym, "HAVE_"))
+					continue;
+				score_file_error(score, f, i+1,
+						 "%s should be tested with #if"
+						 "%s",
+						 sym, explanation);
+				explanation = "";
+			}
+		}
+	}
+
+	if (!score->error) {
+		score->pass = true;
+		score->score = score->total;
+	}
+}
+
+struct ccanlint hash_if = {
+	.key = "hash_if",
+	.name = "Features are checked with #if not #ifdef",
+	.check = check_hash_if,
+	.needs = ""
+};
+
+REGISTER_TEST(hash_if);