make fish buildable on OS X Snow Leopard

I noticed that the `test_convert()` function was randomly failing when
run on OS X Snow Leopard. I tracked it down to the `mbrtowc()` function on
that OS being broken. Explicitly testing for UTF-8 prefixes that identify
a sequence longer than four bytes (which the Unicode standard made illegal
long ago) keeps us from having encoding errors on those OS's.

This also makes the errors reported by the `test_convert()` function actually
useful and readable.

Lastly, it makes it possible to build fish on OS X Snow Leopard.

1 files changed, 8 insertions, 3 deletions

diff --git a/src/fallback.cpp b/src/fallback.cpp
index 7a829124..96fa417b 100644
--- a/src/fallback.cpp
+++ b/src/fallback.cpp
@@ -131,7 +131,8 @@ __attribute__((unused)) static int wcsncasecmp_fallback(const wchar_t *a, const
     return wcsncasecmp_fallback(a + 1, b + 1, count - 1);
 }
 
-#if __APPLE__ && __DARWIN_C_LEVEL >= 200809L
+#if __APPLE__
+#if __DARWIN_C_LEVEL >= 200809L
 // Note parens avoid the macro expansion.
 wchar_t *wcsdup_use_weak(const wchar_t *a) {
     if (&wcsdup != NULL) return (wcsdup)(a);
@@ -147,8 +148,12 @@ int wcsncasecmp_use_weak(const wchar_t *s1, const wchar_t *s2, size_t n) {
     if (&wcsncasecmp != NULL) return (wcsncasecmp)(s1, s2, n);
     return wcsncasecmp_fallback(s1, s2, n);
 }
-
-#endif  //__APPLE__
+#else // __DARWIN_C_LEVEL >= 200809L
+wchar_t *wcsdup(const wchar_t *in) { return wcsdup_fallback(in); }
+int wcscasecmp(const wchar_t *a, const wchar_t *b) { return wcscasecmp_fallback(a, b); }
+int wcsncasecmp(const wchar_t *a, const wchar_t *b, size_t n) { return wcsncasecmp_fallback(a, b, n); }
+#endif // __DARWIN_C_LEVEL >= 200809L
+#endif  // __APPLE__
 
 #ifndef HAVE_WCSNDUP
 wchar_t *wcsndup(const wchar_t *in, size_t c) {