Don't use openttd's internal copy of strcasestr().
Index: src/string_func.h
--- src/string_func.h.orig
+++ src/string_func.h
@@ -256,7 +256,7 @@ inline bool IsWhitespace(char32_t c)
 #endif
 
 /* 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))) || defined(_NETBSD_SOURCE)
+#if defined(_GNU_SOURCE) || (defined(__BSD_VISIBLE) && __BSD_VISIBLE) || (defined(__APPLE__) && (!defined(_POSIX_C_SOURCE) || defined(_DARWIN_C_SOURCE))) || defined(_NETBSD_SOURCE) || defined(__OpenBSD__)
 #	undef DEFINE_STRCASESTR
 #else
 #	define DEFINE_STRCASESTR
