Remove charconv requirement (#142)

* Remove charconv requirement

It's the most restrictive compiler requirement (gcc 8.1, clang 7)

* Use has_include preprocessor function

peglib.h

@@ -11,7 +11,9 @@
 #include <any>
 #include <cassert>
 #include <cctype>
+#if __has_include(<charconv>)
 #include <charconv>
+#endif
 #include <cstring>
 #include <functional>
 #include <initializer_list>
@@ -478,6 +480,7 @@ struct SemanticValues : protected std::vector<std::any> {
 
   template <typename T> T token_to_number() const {
     T n = 0;
+#if __has_include(<charconv>)
     if constexpr (std::is_floating_point<T>::value) {
       // TODO: The following code should be removed eventually.
       std::istringstream ss(token_to_string());
@@ -488,6 +491,11 @@ struct SemanticValues : protected std::vector<std::any> {
       std::from_chars(sv.data(), sv.data() + sv.size(), n);
       return n;
     }
+#else
+    std::istringstream ss(token_to_string());
+    ss >> n;
+    return n;
+#endif
   }
 
   // Transform the semantic value vector to another vector
@@ -3770,6 +3778,7 @@ template <typename Annotation> struct AstBase : public Annotation {
 
   template <typename T> T token_to_number() const {
     T n = 0;
+#if __has_include(<charconv>)
     if constexpr (std::is_floating_point<T>::value) {
       // TODO: The following code should be removed eventually.
       std::istringstream ss(token_to_string());
@@ -3780,6 +3789,11 @@ template <typename Annotation> struct AstBase : public Annotation {
       std::from_chars(token.data(), token.data() + token.size(), n);
       return n;
     }
+#else
+    std::istringstream ss(token_to_string());
+    ss >> n;
+    return n;
+#endif
   }
 };