Port to gcc 4.9.0

There is an issue with GMP, as described at:

https://gcc.gnu.org/gcc-4.9/porting_to.html

section "Header <cstddef> changes". So we just need to make sure the <cstddef>
is always declared before <gmpxx.h>.

Otherwise it compiles and tests run.
Remove duplicate includes of <gmpxx.h>

It must be included in dict.h in order to use mpz_class in STL containers. The
dict.h is then included in basic.h, so the <gmpxx.h> doesn't need to be
included in any other header fil.
Merge pull request #257 from certik/gcc4.9.0

Port to gcc 4.9.0