mpir: Build with C++ support

This is needed by Alkimia which I'm going to package soon as part of
KMyMoney.

I enabled this by default, because it doesn't increase the closure size
a whole lot (only around 150 KiB):

$ nix-store -q --size /nix/store/...old...-mpir-3.0.0
1223248
$ nix-store -q --size /nix/store/...new...-mpir-3.0.0
1377136

Introducing an option for enabling/disabling this is not worth it,
because it doesn't conflict with anything and the size increase is the
only drawback and we can still make it configurable if we want someday.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @7c6f434c