Build error using pre-packaged GMP

[Hans Åberg]

> On 26 May 2021, at 10:31, Marc Glisse <marc.glisse at inria.fr> wrote:
> 
> My first guess would be some misguided
> 
> extern "C" {
> #include <gmp.h>
> }

Indeed it is, testing with g++ 11. The errors that the OP posted arrive at the end, the first, not posted, error is:
/opt/local/include/gcc11/c++/bits/memoryfwd.h:63:3: error: template with C linkage
   63 |   template<typename>
      |   ^~~~~~~~