Fix wcstod_l infinite recursion under FreeBSD

This was the actual issue leading to memory corruption under FreeBSD in
issue #5453, worked around by correcting the detection of `wcstod_l` so
that our version of the function is not called at all.

If we are 100% certain that `wcstod_l` does not exist, then then the
existing code is fine. But given that our checks have failed seperately
on two different platforms already (FreeBSD and Cygwin/newlib), it's a
good precaution to take.

src/fallback.cpp

@@ -390,9 +390,10 @@ int flock(int fd, int op) {
 #endif  // HAVE_FLOCK
 
 #ifndef HAVE_WCSTOD_L
-// musl doesn't feature wcstod_l,
-// so we just wrap wcstod.
-double wcstod_l(const wchar_t *enptr, wchar_t **endptr, locale_t loc) {
+#undef wcstod_l
+// For platforms without wcstod_l C extension, wrap wcstod after changing the
+// thread-specific locale.
+double fish_compat::wcstod_l(const wchar_t *enptr, wchar_t **endptr, locale_t loc) {
     char *saved_locale = strdup(setlocale(LC_NUMERIC, NULL));
     // Yes, this is hardcoded to use the "C" locale.
     // That's the only thing we need, and uselocale(loc) broke in my testing.

src/fallback.h

@@ -200,5 +200,15 @@ int flock(int fd, int op);
 #endif
 
 #ifndef HAVE_WCSTOD_L
-double wcstod_l(const wchar_t *enptr, wchar_t **endptr, locale_t loc);
+// On some platforms if this is incorrectly detected and a system-defined
+// defined version of `wcstod_l` exists, calling `wcstod` from our own
+// `wcstod_l` can call back into `wcstod_l` causing infinite recursion.
+// e.g. FreeBSD defines `wcstod(x, y)` as `wcstod_l(x, y, __get_locale())`.
+// Solution: namespace our implementation to make sure there is no symbol
+// duplication.
+#undef wcstod_l
+namespace fish_compat {
+    double wcstod_l(const wchar_t *enptr, wchar_t **endptr, locale_t loc);
+}
+#define wcstod_l(x, y, z) fish_compat::wcstod_l(x, y, z)
 #endif