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

2 files changed, 198 insertions, 0 deletions

diff --git a/CMake/AbseilHelpers.cmake b/CMake/AbseilHelpers.cmake
new file mode 100644
index 000000000000..f6243c3b4889
--- /dev/null
+++ b/CMake/AbseilHelpers.cmake
@@ -0,0 +1,157 @@
+#
+# 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.
+#
+
+include(CMakeParseArguments)
+
+set(_ABSL_HELPERS_PATH "${CMAKE_CURRENT_LIST_DIR}")
+
+#
+# create a static library absl_based on the following variable
+#
+# parameters
+# SOURCES : sources files for the library
+# PUBLIC_LIBRARIES: targets and flags for linking phase
+# PRIVATE_COMPILE_FLAGS: compile flags for the library. Will not be exported.
+# EXPORT_NAME: export name for the absl:: target export
+# TARGET: target name
+#
+# create a target associated to <NAME>
+# libraries are installed under CMAKE_INSTALL_FULL_LIBDIR by default
+#
+function(absl_library)
+  cmake_parse_arguments(ABSL_LIB
+    "DISABLE_INSTALL" # keep that in case we want to support installation one day
+    "TARGET;EXPORT_NAME"
+    "SOURCES;PUBLIC_LIBRARIES;PRIVATE_COMPILE_FLAGS;PUBLIC_INCLUDE_DIRS;PRIVATE_INCLUDE_DIRS"
+    ${ARGN}
+  )
+
+  set(_NAME ${ABSL_LIB_TARGET})
+  string(TOUPPER ${_NAME} _UPPER_NAME)
+
+  add_library(${_NAME} STATIC ${ABSL_LIB_SOURCES})
+
+  target_compile_options(${_NAME} PRIVATE ${ABSL_COMPILE_CXXFLAGS} ${ABSL_LIB_PRIVATE_COMPILE_FLAGS})
+  target_link_libraries(${_NAME} PUBLIC ${ABSL_LIB_PUBLIC_LIBRARIES})
+  target_include_directories(${_NAME}
+    PUBLIC ${ABSL_COMMON_INCLUDE_DIRS} ${ABSL_LIB_PUBLIC_INCLUDE_DIRS}
+    PRIVATE ${ABSL_LIB_PRIVATE_INCLUDE_DIRS}
+  )
+
+  if(ABSL_LIB_EXPORT_NAME)
+    add_library(absl::${ABSL_LIB_EXPORT_NAME} ALIAS ${_NAME})
+  endif()
+endfunction()
+
+
+
+#
+# header only virtual target creation
+#
+function(absl_header_library)
+  cmake_parse_arguments(ABSL_HO_LIB
+    "DISABLE_INSTALL"
+    "EXPORT_NAME;TARGET"
+    "PUBLIC_LIBRARIES;PRIVATE_COMPILE_FLAGS;PUBLIC_INCLUDE_DIRS;PRIVATE_INCLUDE_DIRS"
+    ${ARGN}
+  )
+
+  set(_NAME ${ABSL_HO_LIB_TARGET})
+
+  set(__dummy_header_only_lib_file "${CMAKE_CURRENT_BINARY_DIR}/${_NAME}_header_only_dummy.cc")
+
+  if(NOT EXISTS ${__dummy_header_only_lib_file})
+    file(WRITE ${__dummy_header_only_lib_file}
+      "\
+       /* generated file for header-only cmake target */ \
+       \
+       // single meaningless symbol \
+       void __${_NAME}__header_fakesym() {} \
+       \
+      "
+    )
+  endif()
+
+
+  add_library(${_NAME} ${__dummy_header_only_lib_file})
+  target_link_libraries(${_NAME} PUBLIC ${ABSL_HO_LIB_PUBLIC_LIBRARIES})
+  target_include_directories(${_NAME}
+    PUBLIC ${ABSL_COMMON_INCLUDE_DIRS} ${ABSL_HO_LIB_PUBLIC_INCLUDE_DIRS}
+    PRIVATE ${ABSL_HO_LIB_PRIVATE_INCLUDE_DIRS}
+  )
+
+  if(ABSL_HO_LIB_EXPORT_NAME)
+    add_library(absl::${ABSL_HO_LIB_EXPORT_NAME} ALIAS ${_NAME})
+  endif()
+
+endfunction()
+
+
+#
+# create an abseil unit_test and add it to the executed test list
+#
+# parameters
+# TARGET: target name prefix
+# SOURCES: sources files for the tests
+# PUBLIC_LIBRARIES: targets and flags for linking phase.
+# PRIVATE_COMPILE_FLAGS: compile flags for the test. Will not be exported.
+#
+# create a target associated to <NAME>_bin
+#
+# all tests will be register for execution with add_test()
+#
+# test compilation and execution is disable when BUILD_TESTING=OFF
+#
+function(absl_test)
+
+  cmake_parse_arguments(ABSL_TEST
+    ""
+    "TARGET"
+    "SOURCES;PUBLIC_LIBRARIES;PRIVATE_COMPILE_FLAGS;PUBLIC_INCLUDE_DIRS"
+    ${ARGN}
+  )
+
+
+  if(BUILD_TESTING)
+
+    set(_NAME ${ABSL_TEST_TARGET})
+    string(TOUPPER ${_NAME} _UPPER_NAME)
+
+    add_executable(${_NAME}_bin ${ABSL_TEST_SOURCES})
+
+    target_compile_options(${_NAME}_bin PRIVATE ${ABSL_COMPILE_CXXFLAGS} ${ABSL_TEST_PRIVATE_COMPILE_FLAGS})
+    target_link_libraries(${_NAME}_bin PUBLIC ${ABSL_TEST_PUBLIC_LIBRARIES} ${ABSL_TEST_COMMON_LIBRARIES})
+    target_include_directories(${_NAME}_bin
+      PUBLIC ${ABSL_COMMON_INCLUDE_DIRS} ${ABSL_TEST_PUBLIC_INCLUDE_DIRS}
+      PRIVATE ${GMOCK_INCLUDE_DIRS} ${GTEST_INCLUDE_DIRS}
+    )
+
+    add_test(${_NAME}_test ${_NAME}_bin)
+  endif(BUILD_TESTING)
+
+endfunction()
+
+
+
+
+function(check_target my_target)
+
+  if(NOT TARGET ${my_target})
+    message(FATAL_ERROR " ABSL: compiling absl requires a ${my_target} CMake target in your project,
+                   see CMake/README.md for more details")
+  endif(NOT TARGET ${my_target})
+
+endfunction()
diff --git a/CMake/README.md b/CMake/README.md
new file mode 100644
index 000000000000..53d3d3351891
--- /dev/null
+++ b/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)
+
+
+
+