CMake: Add support for CMake packages

Summary:
Adds support for CMake packages: https://cmake.org/cmake/help/v3.9/manual/cmake-packages.7.html#creating-packages.

This allow using RocksDB by other CMake projects this way:

```
cmake_minimum_required(VERSION 3.5)
project(rdbt)

find_package(RocksDB CONFIG)

add_executable(rdbt test.cpp)
target_link_libraries(rdbt PRIVATE RocksDB::rocksdb)
```
Closes https://github.com/facebook/rocksdb/pull/2773

Differential Revision: D5722587

Pulled By: sagar0

fbshipit-source-id: 0d90dc4a77b42a617cdbe1348a370e719c282b87

CMakeLists.txt

@@ -648,9 +648,54 @@ if(NOT WIN32 OR ROCKSDB_INSTALL_ON_WINDOWS)
   endif()
 
   include(GNUInstallDirs)
-  install(TARGETS ${ROCKSDB_STATIC_LIB} COMPONENT devel ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR})
-  install(TARGETS ${ROCKSDB_SHARED_LIB} COMPONENT runtime DESTINATION ${CMAKE_INSTALL_LIBDIR})
-  install(DIRECTORY include/rocksdb COMPONENT devel DESTINATION ${CMAKE_INSTALL_INCLUDEDIR})
+  include(CMakePackageConfigHelpers)
+
+  set(package_config_destination ${CMAKE_INSTALL_LIBDIR}/cmake/rocksdb)
+
+  configure_package_config_file(
+    ${CMAKE_SOURCE_DIR}/cmake/RocksDBConfig.cmake.in RocksDBConfig.cmake
+    INSTALL_DESTINATION ${package_config_destination}
+  )
+
+  write_basic_package_version_file(
+    RocksDBConfigVersion.cmake
+    VERSION ${ROCKSDB_VERSION}
+    COMPATIBILITY SameMajorVersion
+  )
+
+  install(DIRECTORY include/rocksdb COMPONENT devel DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}")
+
+  install(
+    TARGETS ${ROCKSDB_STATIC_LIB}
+    EXPORT RocksDBTargets
+    COMPONENT devel
+    ARCHIVE DESTINATION "${CMAKE_INSTALL_LIBDIR}"
+    INCLUDES DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
+  )
+
+  install(
+    TARGETS ${ROCKSDB_SHARED_LIB}
+    EXPORT RocksDBTargets
+    COMPONENT runtime
+    RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}"
+    LIBRARY DESTINATION "${CMAKE_INSTALL_LIBDIR}"
+    INCLUDES DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
+  )
+
+  install(
+    EXPORT RocksDBTargets
+    COMPONENT devel
+    DESTINATION ${package_config_destination}
+    NAMESPACE RocksDB::
+  )
+
+  install(
+    FILES
+    ${CMAKE_CURRENT_BINARY_DIR}/RocksDBConfig.cmake
+    ${CMAKE_CURRENT_BINARY_DIR}/RocksDBConfigVersion.cmake
+    COMPONENT devel
+    DESTINATION ${package_config_destination}
+  )
 endif()
 
 option(WITH_TESTS "build with tests" ON)

cmake/RocksDBConfig.cmake.in

@@ -0,0 +1,3 @@
+@PACKAGE_INIT@
+include("${CMAKE_CURRENT_LIST_DIR}/RocksDBTargets.cmake")
+check_required_components(RocksDB)