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

CMakeLists.txt

@@ -0,0 +1,81 @@
+#
+# Copyright 2017 The Abseil Authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+cmake_minimum_required(VERSION 2.8.12)
+project(absl)
+
+# enable ctest
+include(CTest)
+
+list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/CMake)
+
+include(GNUInstallDirs)
+include(AbseilHelpers)
+
+
+# config options
+set(ABSL_STD_CXX_FLAG "-std=c++11" CACHE STRING "c++ std flag (default: c++11)")
+
+
+
+##
+## Using absl targets
+##
+## all public absl targets are
+## exported with the absl:: prefix
+##
+## e.g absl::base absl::synchronization absl::strings ....
+##
+## DO NOT rely on the internal targets outside of the prefix
+
+
+# include current path
+list(APPEND ABSL_COMMON_INCLUDE_DIRS ${CMAKE_CURRENT_SOURCE_DIR})
+
+# -std=X
+set(CMAKE_CXX_FLAGS "${ABSL_STD_CXX_FLAG} ${CMAKE_CXX_FLAGS}") 
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_WARNING_VLA} ${CMAKE_CXX_FLAGS} ")
+
+
+# find dependencies
+## pthread
+find_package(Threads REQUIRED)
+
+# commented: used only for standalone test
+#add_subdirectory(cctz)
+#add_subdirectory(googletest)
+
+## check targets
+check_target(cctz)
+check_target(gtest)
+check_target(gtest_main)
+check_target(gmock)
+
+# -fexceptions
+set(ABSL_EXCEPTIONS_FLAG "${CMAKE_CXX_EXCEPTIONS}")
+
+# fix stuff
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FIX_MSVC} ${CMAKE_CXX_FLAGS}") 
+
+list(APPEND ABSL_TEST_COMMON_LIBRARIES
+  gtest_main
+  gtest
+  gmock
+  ${CMAKE_THREAD_LIBS_INIT}
+)
+
+add_subdirectory(absl)
+
+