diff --git a/configure.ac b/configure.ac
index 330acc8..8e7b0a8 100644
--- a/configure.ac
+++ b/configure.ac
@@ -356,7 +356,9 @@ AC_ARG_WITH(psl-testfile,
   PSL_TESTFILE="\$(top_srcdir)/list/tests/tests.txt")
 AC_SUBST(PSL_TESTFILE)
 
-AC_CHECK_FUNCS([strndup clock_gettime fmemopen nl_langinfo])
+AC_CHECK_FUNCS([strdup strndup clock_gettime fmemopen nl_langinfo])
+AC_CHECK_DECLS([strdup])
+AC_CHECK_DECLS([localtime_r])
 
 # check for dirent.h
 AC_HEADER_DIRENT
diff --git a/meson.build b/meson.build
index a072988..93eb77d 100644
--- a/meson.build
+++ b/meson.build
@@ -94,7 +94,7 @@ config.set('ENABLE_BUILTIN', enable_builtin)
 config.set('HAVE_UNISTD_H', cc.check_header('unistd.h'))
 config.set('HAVE_STDINT_H', cc.check_header('stdint.h'))
 config.set('HAVE_DIRENT_H', cc.check_header('dirent.h'))
-config.set('HAVE_STRNDUP', cc.has_function('strndup'))
+config.set('HAVE_STRDUP', cc.has_function('strdup'))
 config.set('HAVE_CLOCK_GETTIME', cc.has_function('clock_gettime'))
 config.set('HAVE_FMEMOPEN', cc.has_function('fmemopen'))
 config.set('HAVE_NL_LANGINFO', cc.has_function('nl_langinfo'))
diff --git a/src/psl.c b/src/psl.c
index fa6bfd4..8863ee9 100644
--- a/src/psl.c
+++ b/src/psl.c
@@ -320,6 +320,23 @@ static int suffix_init(psl_entry_t *suffix, const char *rule, size_t length)
 	return 0;
 }
 
+#ifndef HAVE_STRDUP
+static char *strdup(const char *s)
+{
+	char *p = malloc(strlen(s) + 1);
+	if (!p)
+		return NULL;
+	return strcpy(p, s);
+}
+#elif !HAVE_DECL_STRDUP
+/*
+ *  On Linux with
+ *    CC=gcc CFLAGS="-Wall -Wextra -Wpedantic -std=c89" ./configure
+ *  strdup isn't declared (warning: implicit declaration of function 'strdup').
+ */
+char *strdup(const char *);
+#endif
+
 #if !defined(WITH_LIBIDN) && !defined(WITH_LIBIDN2) && !defined(WITH_LIBICU)
 /*
  * When configured without runtime IDNA support (./configure --disable-runtime), we need a pure ASCII
@@ -675,9 +692,9 @@ static int psl_idna_toASCII(psl_idna_t *idna, const char *utf8, char **ascii)
 {
 	int ret = -1;
 
+#if defined(WITH_LIBICU)
 	(void) idna;
 
-#if defined(WITH_LIBICU)
 	/* IDNA2008 UTS#46 punycode conversion */
 	if (idna) {
 		char lookupname_buf[128] = "", *lookupname = lookupname_buf;
@@ -737,6 +754,8 @@ static int psl_idna_toASCII(psl_idna_t *idna, const char *utf8, char **ascii)
 #if IDN2_VERSION_NUMBER >= 0x00140000
 	int rc;
 
+	(void) idna;
+
 	/* IDN2_TRANSITIONAL automatically converts to lowercase
 	 * IDN2_NFC_INPUT converts to NFC before toASCII conversion
 	 * Since IDN2_TRANSITIONAL implicitly does NFC conversion, we don't need
@@ -769,6 +788,8 @@ static int psl_idna_toASCII(psl_idna_t *idna, const char *utf8, char **ascii)
 #elif defined(WITH_LIBIDN)
 	int rc;
 
+	(void) idna;
+
 	if (!utf8_is_valid(utf8)) {
 		/* fprintf(stderr, "Invalid UTF-8 sequence not converted: '%s'\n", utf8); */
 		return -1;
@@ -783,6 +804,8 @@ static int psl_idna_toASCII(psl_idna_t *idna, const char *utf8, char **ascii)
 #else
 	char lookupname[128];
 
+	(void) idna;
+
 	if (domain_to_punycode(utf8, lookupname, sizeof(lookupname)) == 0) {
 		if (ascii)
 			if ((*ascii = strdup(lookupname)))
@@ -835,7 +858,7 @@ static int is_public_suffix(const psl_ctx_t *psl, const char *domain, int type)
 
 	for (p = domain; *p; p++) {
 		if (*p == '.') {
-			if (suffix.nlabels == 255) // weird input, avoid 8bit overflow
+			if (suffix.nlabels == 255) /* weird input, avoid 8bit overflow */
 				return 0;
 			suffix.nlabels++;
 		}
diff --git a/tools/psl.c b/tools/psl.c
index 527d780..564a2e5 100644
--- a/tools/psl.c
+++ b/tools/psl.c
@@ -41,10 +41,15 @@
 
 // Windows does not have localtime_r but has localtime_s, which is more or less
 // the same except that the arguments are reversed
-# define LOCALTIME_R_SUCCESSFUL(t_sec,t_now)	\
+# define LOCALTIME_R_SUCCESSFUL(t_sec, t_now)	\
 	(localtime_s(t_now, t_sec) == 0)
 #else
-# define LOCALTIME_R_SUCCESSFUL(t_sec,t_now)	\
+# include <time.h>
+# if ! HAVE_DECL_LOCALTIME_R
+struct tm *localtime_r(const time_t *, struct tm *);
+#endif
+
+# define LOCALTIME_R_SUCCESSFUL(t_sec, t_now)	\
 	(localtime_r(t_sec, t_now) != NULL)
 #endif