Make _XOPEN_SOURCE definition agree with _POSIX_C_SOURCE for POSIX 2008 env

If we want a POSIX 2008 environment as requested by _POSIX_C_SOURCE here, _XOPEN_SOURCE should be 700 so they are in agreement.

See:
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap02.html#tag_02_01_04

Fixes build on FreeBSD 10 which redefines _POSIX_C_SOURCE depending on _XOPEN_SOURCE def.