**Description**
Added a cmake option to enable Unity builds for libprotobuf-lite.
Added missing symmetric include for port_def.inc
Added preproc guards for PROTOBUF_PRAGMA_INIT_SEG so it does not get included multiple times in unity build.
**Motivation and Context**
For large code base, especially in the game dev industry, Unity builds (https://en.wikipedia.org/wiki/Unity_build) are used to speed up the compilations.
Special care must be taken to enable Unity builds for a project, since many translation units will be merged togheter.
Closes#16454
In CMake >= 3.0 it is more idiomatic to set per-target compiler options than global compiler settings. I have kept these options and defines as `PRIVATE` so they won't be exported with the target.
Closes#12916
jslap-ubi
Joshua Haberman
Carlos O'Ryan