Export of internal Abseil changes.

--
4501f16edb0939be6165df74c70a421d3a959aa5 by Jon Cohen <cohenjon@google.com>:

Remove use of list(FILTER as it's only available in cmake 3.6.  We currently have to support cmake 3.5 at least because of Ubuntu 14.04.  Also make sure to exclude .inc files when checking if the library is header only.

Fixes #263

PiperOrigin-RevId: 235725207

--
68ceb6a674a6cbb74472c64915e9e76d2de2fb8d by Abseil Team <absl-team@google.com>:

Fix comment in FixedArrayTest.SmallObjects.

PiperOrigin-RevId: 235671337
GitOrigin-RevId: 4501f16edb0939be6165df74c70a421d3a959aa5
Change-Id: I7c2759ad9d1d086cc7c213e7964279366113fccb