fixed some small issues

Makefile

 # used programs
 RE2C = re2c
-SED = gsed
+SED = sed
 
 # additional flags
 FLAGS = -Wall -Wextra -pedantic -Weffc++ -Wcast-align -Wcast-qual -Wctor-dtor-privacy -Wdisabled-optimization -Wformat=2 -Winit-self -Wmissing-declarations -Wmissing-include-dirs -Wold-style-cast -Woverloaded-virtual -Wredundant-decls -Wshadow -Wsign-conversion -Wsign-promo -Wstrict-overflow=5 -Wswitch -Wundef -Wno-unused -Wnon-virtual-dtor -Wreorder -Wdeprecated -Wfloat-equal

src/json.hpp

@@ -7,8 +7,8 @@
 @see https://github.com/nlohmann/json
 */
 
-#ifndef _NLOHMANN_JSON
-#define _NLOHMANN_JSON
+#ifndef NLOHMANN_JSON_HPP
+#define NLOHMANN_JSON_HPP
 
 #include <algorithm>
 #include <ciso646>

src/json.hpp.re2c

@@ -7,8 +7,8 @@
 @see https://github.com/nlohmann/json
 */
 
-#ifndef _NLOHMANN_JSON
-#define _NLOHMANN_JSON
+#ifndef NLOHMANN_JSON_HPP
+#define NLOHMANN_JSON_HPP
 
 #include <algorithm>
 #include <ciso646>