[Code] Fix build with older C++ libraries (#4549)

This adds a compile time concept to determine if from_chars has
floating-point support and uses fallbacks if not.

This is a C++17 feature but support for floats was only added to
libstdc++ with GCC 11.1 and LLVM libc++ in 20.0 (unreleased).

common/mysql_stmt.cpp

@@ -414,6 +414,12 @@ static uint64_t MakeBits(std::span<const uint8_t> data)
 	return bits;
 }
 
+template <typename T>
+concept has_from_chars = requires (const char* first, const char* last, T value)
+{
+	std::from_chars(first, last, value);
+};
+
 template <typename T>
 static T FromString(std::string_view sv)
 {
@@ -422,6 +428,14 @@ static T FromString(std::string_view sv)
 		// return false for empty (zero-length) strings
 		return !sv.empty();
 	}
+	else if constexpr (std::is_same_v<T, float> && !has_from_chars<T>)
+	{
+		return std::strtof(std::string(sv).c_str(), nullptr);
+	}
+	else if constexpr (std::is_same_v<T, double> && !has_from_chars<T>)
+	{
+		return std::strtod(std::string(sv).c_str(), nullptr);
+	}
 	else
 	{
 		// non numbers return a zero initialized T (could return nullopt instead)