Correct fish_wcstod and fix Linux build failure

Limit the fish_wcstod fast path to ASCII digits only, to fix the problem
observed in the discussion for a700acadfa
where LANG=de_DE.UTF-8 would cause `test` to interpret commas instead of
periods inside floating point values.

src/fish_tests.cpp

@@ -6,6 +6,7 @@
 #include <errno.h>
 #include <libgen.h>
 #include <limits.h>
+#include <math.h>
 #include <pthread.h>
 #include <signal.h>
 #include <stdarg.h>

src/wutil.cpp

@@ -705,10 +705,12 @@ unsigned long long fish_wcstoull(const wchar_t *str, const wchar_t **endptr, int
 double fish_wcstod(const wchar_t *str, wchar_t **endptr) {
     // The "fast path." If we're all ASCII and we fit inline, use strtod().
     char narrow[128];
-    size_t len_plus_0 = 1 + wcslen(str);
-    auto is_ascii = [](wchar_t c) { return c >= 0 && c <= 127; };
-    if (len_plus_0 <= sizeof narrow && std::all_of(str, str + len_plus_0, is_ascii)) {
+    size_t len = wcslen(str);
+    size_t len_plus_0 = 1 + len;
+    auto is_digit = [](wchar_t c) { return '0' <= c && c <= '9'; };
+    if (len_plus_0 <= sizeof narrow && std::all_of(str, str + len, is_digit)) {
         // Fast path. Copy the string into a local buffer and run strtod() on it.
+        // We can ignore the locale-taking version because we are limited to ASCII digits.
         std::copy(str, str + len_plus_0, narrow);
         char *narrow_endptr = nullptr;
         double ret = strtod(narrow, endptr ? &narrow_endptr : nullptr);