complex.h causes havoc, e.g.
```
libcomplexDict.rootmap:2:13: error: declaration of anonymous class must be a definition
template <> class complex<float>;
```
running roottest_root_tree_cloning_treeCloneTest, which is caused by `complex` being redefined
to _Complex, which is a kind of type annotation and breaks uses of `std::complex`.

This means we can now remove `#undef I` (which comes from complex.h).
Use cute raw string literals and combine remaining `#undef`s into a single call to `declare()`