ARROW-11740: [C++] posix_memalign not declared in scope on Solaris
ARROW-11740: [C++] posix_memalign not declared in scope on Solaris

I've confirmed that this compiles on Solaris.

Closes #9694 from nealrichardson/posix-memalign

Lead-authored-by: Neal Richardson <beb59f0141269d2d04e73d0dcc1967b64ba87e13@gmail.com>
Co-authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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