Assimp master head fixes for failure to compile (#5899)

* Fix unknown pragma error on msys2 mingw

* Fix missing assignment operator on msys2 mingw

Default is omitted by compiler because custom copy constructor

* Fix redefinition of _1 and _2 error on msys2 mingw

fix is converting it to lambda as clang is claiming that is
preferred

* Fix strncpy warnings about truncation

* Fix missing assignment operator error on clang

* Update glTFImporter.cpp

* Update glTF2Importer.cpp

---------

Co-authored-by: Kim Kulling <kimkulling@users.noreply.github.com>

code/Common/Maybe.h

@@ -83,6 +83,7 @@ struct Maybe {
 
     Maybe &operator&() = delete;
     Maybe(const Maybe &) = delete;
+    Maybe &operator=(const Maybe &) = default;
 
 private:
     T _val;