Define __EXTENSIONS__ on Solaris, too.

Define __EXTENSIONS__ on Solaris, too.

Apparently, if you define _POSIX_C_SOURCE on Solaris,
that's interpreted as "you get ONLY what's defined by POSIX".
Results from BF member hake show that that breaks perl.h,
and doubtless it'd cause more problems if we got past that.
Adopt the suggestion from standards(7) that we also need to
define __EXTENSIONS__, in hopes of un-breaking things.

Discussion: https://postgr.es/m/1654508.1733162761@sss.pgh.pa.us