Be more explicit about C++ version requierments. (#311)

* Be more explicit about C++ version requierments.

* Typo.

Author: bcsgh

README.md

@@ -79,6 +79,7 @@ libhttpserver can be used without any dependencies aside from libmicrohttpd.
 
 The minimum versions required are:
 * g++ >= 5.5.0 or clang-3.6
+* C++17 or newer
 * libmicrohttpd >= 0.9.64
 * [Optionally]: for TLS (HTTPS) support, you'll need [libgnutls](http://www.gnutls.org/).
 * [Optionally]: to compile the code-reference, you'll need [doxygen](http://www.doxygen.nl/).

src/httpserver.hpp

@@ -21,6 +21,10 @@
 #ifndef SRC_HTTPSERVER_HPP_
 #define SRC_HTTPSERVER_HPP_
 
+#if __cplusplus < 201703L
+#  error("libhttpserver requires C++17 or later.")
+#endif
+
 #define _HTTPSERVER_HPP_INSIDE_
 
 #include "httpserver/basic_auth_fail_response.hpp"