compile gtest only when enable test

Summary: Pull Request resolved: https://github.com/facebook/rocksdb/pull/5248

Differential Revision: D15149190

Pulled By: maysamyabandeh

fbshipit-source-id: fd6d799e80bb502a7ddbc07032ea87e2e3f1e24f

CMakeLists.txt

@@ -461,8 +461,6 @@ include_directories(${PROJECT_SOURCE_DIR}/include)
 include_directories(SYSTEM ${PROJECT_SOURCE_DIR}/third-party/gtest-1.7.0/fused-src)
 find_package(Threads REQUIRED)
 
-add_subdirectory(third-party/gtest-1.7.0/fused-src/gtest)
-
 # Main library source code
 
 set(SOURCES
@@ -843,6 +841,7 @@ endif()
 
 option(WITH_TESTS "build with tests" ON)
 if(WITH_TESTS)
+  add_subdirectory(third-party/gtest-1.7.0/fused-src/gtest)
   set(TESTS
         cache/cache_test.cc
         cache/lru_cache_test.cc