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

README.md

@@ -10,6 +10,7 @@ standard library.
 - [Codemap](#codemap)
 - [License](#license)
 - [Links](#links)
+- [Build with cmake](#cmake)
 
 <a name="about"></a>
 ## About Abseil
@@ -85,6 +86,11 @@ For more information about Abseil:
   [Abseil Compatibility Guarantees](http://abseil.io/about/compatibility) to
   understand both what we promise to you, and what we expect of you in return.
 
+<a name="cmake"></a>
+## Build with CMake
+
+Please check the [CMake build instructions](CMake/README.md)
+
 ## Disclaimer
 
 *   This is not an official Google product.