Abseil CMake build instructions

Recommended usage : incorporate Abseil into an CMake project

We recommended to build and use abseil in the same way than googletest
( https://github.com/google/googletest/blob/master/googletest/README.md )

* Download abseil and copy it in a sub-directory in your project.

* Or add abseil as a git-submodule in your project

You can then use the cmake `add_subdirectory()` command to include
abseil directly and use the abseil targets in your project.

Abseil requires CCTZ and the googletest framework. Consequently, 
the targets  `gtest`, `gtest_main`, `gmock` and `cctz` need
to be declared in your project before including abseil with `add_subdirectory`. 
You can find instructions on how to get and build these projects at these 
URL :
    * cctz  https://github.com/google/cctz
    * googletest https://github.com/google/googletest



Here is a short CMakeLists.txt example of a possible project file 
using abseil

project(my_project)

add_subdirectory(googletest)
add_subdirectory(cctz)    
add_subdirectory(abseil-cpp)

add_executable(my_exe source.cpp)
target_link_libraries(my_exe base synchronization strings)