- c3a608de577e0c278b50916ad4803549929f8f72 Merging https://github.com/abseil/abseil-cpp/pull/8/ inte... by Gennadiy Civil <misterg@google.com>
  - d0b528cdf5843db871784c629cb4e7c5165af716 explicitly cast -1 for Span::npos by Jon Cohen <cohenjon@google.com>
  - 32066311a4379f1144f029aaa3740af59b1e364e Remove GUARDED_VAR and PT_GUARDED_VAR entirely. by Abseil Team <absl-team@google.com>
  - 3d3c69d97d15b5c6457906631054109094c083a6 Remove unneeded inline on constexpr definitions. by Alex Strelnikov <strel@google.com>
  - a9a8fe71f90d0b80de8e77375228a7185032636b Remove unneeded lint suppression. by Alex Strelnikov <strel@google.com>

GitOrigin-RevId: c3a608de577e0c278b50916ad4803549929f8f72
Change-Id: I0897ce0b11e41f83fed8d88f18e079a15d086527

- initial cmake support
- downgrade cmake requirement to 2.8.12
- factorize cmake test flags / libs options
- refactor test / library under helpers functions, follow bazel's style
- Add fix for MSVC and Windows support ( thx @patrikfors )
- Switch to default "add_subdirectory()" usage mode
- add CMake/README.md for instructions
- add header-only cmake target generator
- map absl target to absl:: namespace