got rid of std::invoke to avoid C++17 dependence

include/amsmathutil25/util/amsmathutil25_utilimpl.hpp

@@ -18,12 +18,16 @@ template<typename callable, typename ... argst> int threaded_execute(callable &&
     {
         nthreads = 1;
         I = 0;
-        std::invoke(
-            std::forward<callable>(fptr),
-            I,
-            nthreads,
-            std::forward<argst>(args)...
-        );
+        // std::invoke(
+        //     std::forward<callable>(fptr),
+        //     I,
+        //     nthreads,
+        //     std::forward<argst>(args)...
+        // );
+
+        //std::invoke is a C++17 feature, and mingw8 complains even so.
+        // Can I get away with just calling the functions?
+        fptr(I,nthreads,std::forward<argst>(args)...);
     }
     else
     {