build: Use newer version of X/Open system interface

Defining _XOPEN_SOURCE to 500 on FreeBSD means a C89 environment with
_POSIX_C_SOURCE set to 199506, which disables C99 functions. Since
ceilf is a new function added in C99, we should set _XOPEN_SOURCE to a
higher value.

https://bugzilla.gnome.org/show_bug.cgi?id=783431