Export of internal Abseil changes

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932b2203beb5cf08a2b77190ff43fa029c968ad6 by Andy Soffer <asoffer@google.com>:

Updating CMake documentation.

PiperOrigin-RevId: 268509936

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fd9c53b9edda77e7b01189c2e3babc993e404f3f by Abseil Team <absl-team@google.com>:

Rename flags "internal" build target to "flag_internal".

PiperOrigin-RevId: 268485876
GitOrigin-RevId: 932b2203beb5cf08a2b77190ff43fa029c968ad6
Change-Id: Ic9efdc920925fa3c97e8cd4281b86f710f996c14