fix a macro

This prevents gcc 4.9.0 from printing this error message:

     error: right-hand operand of comma expression has no effect [-Werror=unused-value]
fix an include file bug

Through some strange misfortune, gcc 4.9.0 reveals a bug in the c++
include file cstddef, which fails to properly include stddef.h first.
So every where in e/ where gmp.h is included, we first include stddef.h.
my Makefile