fix #162 (#163)

* fix unreachable code warning when <charconv> header is present

* update macro name

peglib.h

@@ -13,6 +13,9 @@
 #include <cctype>
 #if __has_include(<charconv>)
 #include <charconv>
+#define CPPPEGLIB_HAS_CHARCONV true
+#else
+#define CPPPEGLIB_HAS_CHARCONV false
 #endif
 #include <cstring>
 #include <functional>
@@ -340,15 +343,13 @@ inline std::string resolve_escape_sequence(const char *s, size_t n) {
 
 template <typename T> T token_to_number_(std::string_view sv) {
   T n = 0;
-#if __has_include(<charconv>)
-  if constexpr (!std::is_floating_point<T>::value) {
+  if constexpr (CPPPEGLIB_HAS_CHARCONV && !std::is_floating_point<T>::value) {
     std::from_chars(sv.data(), sv.data() + sv.size(), n);
-    return n;
+  } else {
+    auto s = std::string(sv);
+    std::istringstream ss(s);
+    ss >> n;
   }
-#endif
-  auto s = std::string(sv);
-  std::istringstream ss(s);
-  ss >> n;
   return n;
 }