build: add -D_POSIX_C_SOURCE=200809L

[gperciva]

Without this, clang-3.8 with -std=c99 on Ubuntu 16.04 fails to find SSIZE_MAX
in limits.h. This is because SSIZE_MAX is in .../bits/posix1_lim.h, and
/usr/include/limits.h contains:

#ifdef  __USE_POSIX
/* POSIX adds things to <limits.h>.  */
# include <bits/posix1_lim.h>
#endif

and in "STRICT_ANSI" mode (implied by -std=c99, amongst other things),
__USE_POSIX is only defined if we've defined _POSIX_C_SOURCE or _XOPEN_SOURCE.

[gperciva]

This is copied from Tarsnap/scrypt#55

[cperciva]

I'm slightly worried that this may break other platforms, by convincing them to hide non-POSIX functions... I'm going to merge it for now, but be aware that there may be fallout later.

[gperciva]

Hmm? SSIZE_MAX is notated as being [CX]:

[CX] Some of the functionality described on this reference page extends the ISO C standard. Applications shall define the appropriate feature test macro (see XSH The Compilation Environment ) to enable the visibility of these symbols in this header.
...
[CX]  Extension to the ISO C standard 
The functionality described is an extension to the ISO C standard. Application developers may make use of an extension as it is supported on all POSIX.1-2008-conforming systems.
http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/limits.h.html

So it makes sense to me that we need to use -D_POSIX_C_SOURCE=200809L. We use it in cperciva software... I feel like I'm missing something here?