CMake support

- 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

CMake/README.md

@@ -0,0 +1,41 @@
+
+## 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)
+
+
+
+