Set _XOPEN_SOURCE to 700 for FreeBSD.

[iphydf]

This change is 

[iphydf]

Let's do this quick-fix for now, and figure out the correct way to do enable things later.

[robinlinden]

Reviewed 1 of 1 files at r1.
Reviewable status: complete! 1 of 1 LGTMs obtained

[Low-power]

This break build on Solaris

/bin/sh ../libtool  --tag=CC   --mode=compile gcc-4.3.2 -std=gnu99 -DHAVE_CONFIG_H -I. -I..   -I/usr/local/include -I.. -I../toxcore   -D_REENTRANT -pthreads -g -O2 -MT ../toxcore/libtoxcore_la-network.lo -MD -MP -MF ../toxcore/.deps/libtoxcore_la-network.Tpo -c -o ../toxcore/libtoxcore_la-network.lo `test -f '../toxcore/network.c' || echo './'`../toxcore/network.c
libtool: compile:  gcc-4.3.2 -std=gnu99 -DHAVE_CONFIG_H -I. -I.. -I/usr/local/include -I.. -I../toxcore -D_REENTRANT -pthreads -g -O2 -MT ../toxcore/libtoxcore_la-network.lo -MD -MP -MF ../toxcore/.deps/libtoxcore_la-network.Tpo -c ../toxcore/network.c  -fPIC -DPIC -o ../toxcore/.libs/libtoxcore_la-network.o
In file included from /usr/include/sys/int_types.h:55,
                 from /usr/include/sys/stdint.h:38,
                 from /usr/include/stdint.h:38,
                 from ../toxcore/logger.h:27,
                 from ../toxcore/network.h:27,
                 from ../toxcore/network.c:74:
/usr/lib/gcc/i386-pc-solaris2.11/4.3.2/include-fixed/sys/feature_tests.h:362:2: error: #error "Compiler or options invalid for pre-UNIX 03 X/Open applications  and pre-2001 POSIX applications"
make[2]: *** [../toxcore/libtoxcore_la-network.lo] Error 1
make[2]: Leaving directory `/export/home/WHR/src/toxcore-efff822/build'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/export/home/WHR/src/toxcore-efff822'
make: *** [all] Error 2

Macro _XOPEN_SOURCE can set to no more than 600 at this system.

[iphydf]

@Low-power thanks for pointing that out, I'll see about porting to Solaris again soon.