Define __EXTENSIONS__ on illumos

[jasonbking]

This allows the core library to build on illumos (the C++ benchmarks don't build due to a different issue surrounding the use of the restrict keyword that I can file).

[lemire]

Can you explain this PR? I am not sure that I understand.

[jasonbking]

There's apparently a bug on illumos platforms that prevents the various int macros (PRIu32, etc) from being visible when using _XOPEN_SOURCE 700 (I'm still waiting for feedback to confirm this, though my suspicion it is).

Unless you only ever want the exact set of symbols defined by a particular POSIX or X/Open standard, defining __EXTENSIONS__ gives you all the things you typically expect that aren't strictly in either standard, but are still largely universal.

[lemire]

Merging.