conditionally remove long-deprecated in C++ "register" in types

[dimpase]

See #6 for a better fix.

in particular, as Sage moves to C++17, it becomes crucial. And "register" has purely nostalgic meaning nowadays, anyway

[malb]

Is Sage compiling M4RIE as C++ and not as C?

[dimpase]

Sage does the default thing when building m4rie. The error I see happens when building the interface to it, on macOS with a recent xcode, something that used Apple clang 16 or 17 (it enforces C++17 by default). The error goes away if I remove register in its headers.

[dimpase]

Perhaps it's actually a strictly Sage bug - for some strange reason src/sage/libs/m4ri.pxd has # distutils: language = c++ - which probably induces C++ for m4rie.pxd as well, as it's included in the latter, and as a result m4rie extension is built as C++ extension.

Let me try to remove that # distutils: and see how it goes without.

[dimpase]

OK, I'll change this to something which only touches one header file, and is conditional on being compiled with C++.