CMake OSX rpath management (#4620)

* CMake: Add comment for CMP0048

* CMake: osx use @rpath/ as target's install name (CMP0042)

On MacoS library should use @rpath/ as prefix path instead of absolute build path
e.g. otool -L libprotobuf.dylib
libprotobuf.dylib:
  @rpath/libprotobuf.dylib (...)
  ...

* CMake: add rpath to target for LINUX and APPLE

cmake/CMakeLists.txt

@@ -7,7 +7,11 @@ endif()
 
 # CMake policies
 cmake_policy(SET CMP0022 NEW)
-
+# On MacOS use @rpath/ for target's install name prefix path
+if (POLICY CMP0042)
+  cmake_policy(SET CMP0042 NEW)
+endif ()
+# Clear VERSION variables when no VERSION is given to project()
 if(POLICY CMP0048)
   cmake_policy(SET CMP0048 NEW)
 endif()

cmake/install.cmake

@@ -15,6 +15,13 @@ foreach(_library ${_protobuf_libraries})
     PROPERTY INTERFACE_INCLUDE_DIRECTORIES
     $<BUILD_INTERFACE:${protobuf_source_dir}/src>
     $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
+  if (UNIX AND NOT APPLE)
+    set_property(TARGET ${_library}
+      PROPERTY INSTALL_RPATH "$ORIGIN")
+  elseif (APPLE)
+    set_property(TARGET ${_library}
+      PROPERTY INSTALL_RPATH "@loader_path")
+  endif()
   install(TARGETS ${_library} EXPORT protobuf-targets
     RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR} COMPONENT ${_library}
     LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR} COMPONENT ${_library}
@@ -24,6 +31,13 @@ endforeach()
 if (protobuf_BUILD_PROTOC_BINARIES)
   install(TARGETS protoc EXPORT protobuf-targets
     RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR} COMPONENT protoc)
+  if (UNIX AND NOT APPLE)
+    set_property(TARGET protoc
+      PROPERTY INSTALL_RPATH "$ORIGIN/../lib")
+  elseif (APPLE)
+    set_property(TARGET protoc
+      PROPERTY INSTALL_RPATH "@loader_path/../lib")
+  endif()
 endif (protobuf_BUILD_PROTOC_BINARIES)
 
 install(FILES ${CMAKE_CURRENT_BINARY_DIR}/protobuf.pc ${CMAKE_CURRENT_BINARY_DIR}/protobuf-lite.pc DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")