[WIP] CMake Install rule proposal

[Mizux]

FYI to "Discuss and review the changes in this comparison with others."
Add install rule to fix #111

• Add install rule to absl_cc_library() function
• Add abslConfig.cmake

note: This is what I would use to integrate abseil-cpp to our Makefile-based/CMake-based build of https://github.com/google/or-tools (cf google/or-tools#871)

note: For Googlers, backported (and pending) as CL 225323703

[JonathanDCohen]

In the interest of incremental change, can we wait until #136 is in, and then add this to absl_cc_library?

[Mizux]

I think first we should use absl_cc_library() everywhere since the merge of #136
Currently on master:

[0]─[~/work/abseil-cpp]
[^v^]─corentinl@corentinl %grepc -RIin "absl_\(library\|header\)" absl 
absl/meta/CMakeLists.txt:32:absl_header_library(
absl/types/CMakeLists.txt:28:absl_header_library(
absl/types/CMakeLists.txt:43:absl_header_library(
absl/types/CMakeLists.txt:59:absl_library(
absl/types/CMakeLists.txt:75:absl_library(
absl/types/CMakeLists.txt:94:absl_library(
absl/types/CMakeLists.txt:106:absl_library(
absl/algorithm/CMakeLists.txt:34:absl_header_library(
absl/memory/CMakeLists.txt:22:absl_header_library(
absl/container/CMakeLists.txt:51:absl_header_library(
absl/container/CMakeLists.txt:70:absl_library(
absl/utility/CMakeLists.txt:22:absl_header_library(
absl/strings/CMakeLists.txt:72:absl_library(
absl/strings/CMakeLists.txt:84:absl_header_library(
absl/strings/CMakeLists.txt:94:absl_library(
absl/strings/CMakeLists.txt:121:absl_library(
absl/debugging/CMakeLists.txt:72:absl_library(
absl/debugging/CMakeLists.txt:81:absl_library(
absl/debugging/CMakeLists.txt:93:absl_library(
absl/debugging/CMakeLists.txt:106:absl_library(
absl/debugging/CMakeLists.txt:121:absl_library(
absl/debugging/CMakeLists.txt:134:absl_header_library(
absl/debugging/CMakeLists.txt:152:absl_library(
absl/numeric/CMakeLists.txt:27:absl_library(
absl/numeric/CMakeLists.txt:39:absl_header_library(
absl/hash/CMakeLists.txt:36:absl_library(
absl/synchronization/CMakeLists.txt:50:absl_library(
absl/time/CMakeLists.txt:60:absl_library(
absl/base/CMakeLists.txt:74:absl_library(
absl/base/CMakeLists.txt:124:absl_library(
[0]─[~/work/abseil-cpp]
[^v^]─corentinl@corentinl %grepc -RIin "absl_cc_library(" absl
absl/base/CMakeLists.txt:86:absl_cc_library(
absl/base/CMakeLists.txt:101:absl_cc_library(
absl/base/CMakeLists.txt:131:absl_cc_library(
absl/base/CMakeLists.txt:141:absl_cc_library(

EDIT: done internally and make their way outside...

[dennisklein]

It would be nice to have more meta data here, e.g.

set_and_check(@PROJECT_NAME@_HOME "${PACKAGE_PREFIX_DIR}")
set_and_check(@PROJECT_NAME@_INCLUDEDIR "${PACKAGE_PREFIX_DIR}/@CMAKE_INSTALL_INCLUDEDIR@")
set_and_check(@PROJECT_NAME@_LIBDIR "${PACKAGE_PREFIX_DIR}/@CMAKE_INSTALL_LIBDIR@")

set(@PROJECT_NAME@_TARGETS absl::* absl::*_internal)
set(@PROJECT_NAME@_PUBLIC_TARGETS absl::*)
set(@PROJECT_NAME@_PRIVATE_TARGETS absl::*_internal)

The paths may be extracted from the target properties, but experience shows, it is nicer to have them at hand in a convenient variable (${PACKAGE_PREFIX_DIR} is populated by the code generated via @PACKAGE_INIT@ at import time).

The entries in the proposed target lists could e.g. easily be collected in the absl_cc_library function.

Note: @PROJECT_NAME@ is case-sensitive. You may wish to convert to upper case characters.

Note: If you decide to export a path via set_and_check as proposed in my example above, make sure, this is done only in the cases, when something is installed into these paths. E.g. if you had a build option, which would result in nothing being installed into the lib directory, it is likely the lib directory will not be created and set_and_check will fail.

[dennisklein]

To support shared libraries (Please ignore, if this is not a desired feature at all):

Suggested change

	add_library(${_NAME} STATIC "")
	add_library(${_NAME})

The user can now choose between static and shared libraries via -DBUILD_SHARED_LIBS=ON/OFF. For good practice, you may want to explicitely declare this build switch with an explicit default via option(). Without declaring it as an option, the default is OFF (=> static).

Because there are library dependencies within the absl package, it is recommended to add relative RPATH entries for relocatability. Here is a snippet how we usually add such entries on Linux and MacOS:

set(CMAKE_INSTALL_RPATH_USE_LINK_PATH TRUE)
list(FIND CMAKE_PLATFORM_IMPLICIT_LINK_DIRECTORIES "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}" isSystemDir)
if("${isSystemDir}" STREQUAL "-1")
  if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
    set(CMAKE_EXE_LINKER_FLAGS ${CMAKE_EXE_LINKER_FLAGS} "-Wl,--enable-new-dtags")
    set(CMAKE_SHARED_LINKER_FLAGS ${CMAKE_SHARED_LINKER_FLAGS} "-Wl,--enable-new-dtags")
    set(CMAKE_INSTALL_RPATH "$ORIGIN/../${CMAKE_INSTALL_LIBDIR}")
  elseif(CMAKE_SYSTEM_NAME STREQUAL "Darwin")
    set(CMAKE_INSTALL_RPATH "@loader_path/../${CMAKE_INSTALL_LIBDIR}")
  endif()
endif()

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[googlebot]

CLAs look good, thanks!