Fix define in arith.h

arith_dedekind_sum_coprime_d had an incorrect define, which made
tests with --enable-cxx fail.

With this, my #82 patch, gmp 6.0.0, mpfr 3.1.2.p8 and gcc 4.9.0 on an x86_64 system, make check with --enable-cxx completes successfully.