Apply LTS transformations for 20210324 LTS branch (#920)

* Apply LTS transformations for 20210324 LTS branch

* Change the SOVERSION to make MacOS happy

MacOS expects the first part of the SOVERSION to fit into 16 bits.

CMake/AbseilHelpers.cmake

@@ -263,7 +263,7 @@ Cflags: -I\${includedir}${PC_CFLAGS}\n")
     if(ABSL_ENABLE_INSTALL)
       set_target_properties(${_NAME} PROPERTIES
         OUTPUT_NAME "absl_${_NAME}"
-        SOVERSION 0
+        SOVERSION "2103.0.0"
       )
     endif()
   else()

CMakeLists.txt

@@ -45,7 +45,7 @@ endif (POLICY CMP0077)
 # This must come before the project() and include(CTest) lines.
 OPTION(BUILD_TESTING "Build tests" OFF)
 
-project(absl LANGUAGES CXX)
+project(absl LANGUAGES CXX VERSION 20210324)
 include(CTest)
 
 # Output directory is correct by default for most build setups. However, when
@@ -144,17 +144,7 @@ endif()
 add_subdirectory(absl)
 
 if(ABSL_ENABLE_INSTALL)
-  # absl:lts-remove-begin(system installation is supported for LTS releases)
-  # We don't support system-wide installation
-  list(APPEND SYSTEM_INSTALL_DIRS "/usr/local" "/usr" "/opt/" "/opt/local" "c:/Program Files/${PROJECT_NAME}")
-  if(NOT DEFINED CMAKE_INSTALL_PREFIX OR CMAKE_INSTALL_PREFIX IN_LIST SYSTEM_INSTALL_DIRS)
-    message(WARNING "\
-  The default and system-level install directories are unsupported except in LTS \
-  releases of Abseil.  Please set CMAKE_INSTALL_PREFIX to install Abseil in your \
-  source or build tree directly.\
-    ")
-  endif()
-  # absl:lts-remove-end
+  
 
   # install as a subdirectory only
   install(EXPORT ${PROJECT_NAME}Targets

absl/base/options.h

@@ -205,8 +205,8 @@
 // be changed to a new, unique identifier name.  In particular "head" is not
 // allowed.
 
-#define ABSL_OPTION_USE_INLINE_NAMESPACE 0
-#define ABSL_OPTION_INLINE_NAMESPACE_NAME head
+#define ABSL_OPTION_USE_INLINE_NAMESPACE 1
+#define ABSL_OPTION_INLINE_NAMESPACE_NAME lts_20210324
 
 // ABSL_OPTION_HARDENED
 //