Make FindZLIB consistent with official definitions (#4823)

Summary:
CMake 3 already has FindZLIB.
[https://cmake.org/cmake/help/v3.13/module/FindZLIB.html](https://cmake.org/cmake/help/v3.13/module/FindZLIB.html)
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4823

Differential Revision: D13567653

Pulled By: ajkr

fbshipit-source-id: e424aac1e5d9af4ee0d293896faedf7c712f7734

CMakeLists.txt

@@ -94,7 +94,13 @@ else()
   if(WITH_ZLIB)
     find_package(zlib REQUIRED)
     add_definitions(-DZLIB)
-    include_directories(${ZLIB_INCLUDE_DIR})
+    if(ZLIB_INCLUDE_DIRS)
+      # CMake 3
+      include_directories(${ZLIB_INCLUDE_DIRS})
+    else()
+      # CMake 2
+      include_directories(${ZLIB_INCLUDE_DIR})
+    endif()
     list(APPEND THIRDPARTY_LIBS ${ZLIB_LIBRARIES})
   endif()
 

cmake/modules/Findzlib.cmake

@@ -1,21 +0,0 @@
-# - Find zlib
-# Find the zlib compression library and includes
-#
-# ZLIB_INCLUDE_DIR - where to find zlib.h, etc.
-# ZLIB_LIBRARIES - List of libraries when using zlib.
-# ZLIB_FOUND - True if zlib found.
-
-find_path(ZLIB_INCLUDE_DIR
-  NAMES zlib.h
-  HINTS ${ZLIB_ROOT_DIR}/include)
-
-find_library(ZLIB_LIBRARIES
-  NAMES z
-  HINTS ${ZLIB_ROOT_DIR}/lib)
-
-include(FindPackageHandleStandardArgs)
-find_package_handle_standard_args(zlib DEFAULT_MSG ZLIB_LIBRARIES ZLIB_INCLUDE_DIR)
-
-mark_as_advanced(
-  ZLIB_LIBRARIES
-  ZLIB_INCLUDE_DIR)