#include <optional> if C++ version >= 17 (#122)

Author: Vipul-Cariappa

clingwrapper/src/clingwrapper.cxx

@@ -250,6 +250,9 @@ class ApplicationStarter {
                "#include <set>\n" // FIXME: Replace with modules
                "#include <chrono>\n" // FIXME: Replace with modules
                "#include <cmath>\n" // FIXME: Replace with modules
+               "#if __has_include(<optional>)\n"
+                   "#include <optional>\n"
+               "#endif\n"
                "#include \"clang/Interpreter/CppInterOp.h\"";
         Cpp::Process(code);