diff --git a/src/string.cpp b/src/string.cpp
index 289031cfc7..1ab68a2744 100644
--- a/src/string.cpp
+++ b/src/string.cpp
@@ -362,7 +362,9 @@ char *strndup(const char *s, size_t len)
 	memcpy(tmp, s, len);
 	return tmp;
 }
+#endif /* !_GNU_SOURCE */
 
+#ifdef DEFINE_STRCASESTR
 const char *strcasestr(const char *haystack, const char *needle)
 {
 	size_t hay_len = strlen(haystack);
@@ -376,4 +378,4 @@ const char *strcasestr(const char *haystack, const char *needle)
 
 	return NULL;
 }
-#endif /* !_GNU_SOURCE */
+#endif /* DEFINE_STRCASESTR */
diff --git a/src/string_func.h b/src/string_func.h
index c63fc6719b..ad32a55587 100644
--- a/src/string_func.h
+++ b/src/string_func.h
@@ -234,9 +234,16 @@ static inline bool IsWhitespace(WChar c)
 }
 
 #ifndef _GNU_SOURCE
-/* strndup and strcasestr are GNU extensions */
+/* strndup is a GNU extension */
 char *strndup(const char *s, size_t len);
-const char *strcasestr(const char *haystack, const char *needle);
 #endif /* !_GNU_SOURCE */
 
+/* strcasestr is available for _GNU_SOURCE, BSD and some Apple */
+#if defined(_GNU_SOURCE) || (defined(__BSD_VISIBLE) && __BSD_VISIBLE) || (defined(__APPLE__) && (!defined(_POSIX_C_SOURCE) || defined(_DARWIN_C_SOURCE)))
+#	undef DEFINE_STRCASESTR
+#else
+#	define DEFINE_STRCASESTR
+const char *strcasestr(const char *haystack, const char *needle);
+#endif /* strcasestr is available */
+
 #endif /* STRING_FUNC_H */