Add dependency of gtest on pthread (#6572)

Summary:
Compilation of rocksdb fails because -lpthread flag is needed by gtest

**Before modification** :
/usr/bin/c++   -W -Wextra -Wall -Wsign-compare -Wshadow -Wno-unused-parameter -Wno-unused-variable -Woverloaded-virtual -Wnon-virtual-dtor -Wno-missing-field-initializers -Wno-strict-aliasing -std=c++11 -march=native -Werror -fno-builtin-memcmp -g -DROCKSDB_USE_RTTI   CMakeFiles/table_reader_bench.dir/table/table_reader_bench.cc.o  -o table_reader_bench -Wl,-rpath,/develop/src/rocksdb/build librocksdb.so.6.8.0 libtestharness.a /usr/lib/x86_64-linux-gnu/libgflags.so -lpthread third-party/gtest-1.8.1/fused-src/gtest/libgtest.a

**After modification** :
/usr/bin/c++   -W -Wextra -Wall -Wsign-compare -Wshadow -Wno-unused-parameter -Wno-unused-variable -Woverloaded-virtual -Wnon-virtual-dtor -Wno-missing-field-initializers -Wno-strict-aliasing -std=c++11 -march=native -Werror -fno-builtin-memcmp -g -DROCKSDB_USE_RTTI   CMakeFiles/table_reader_bench.dir/table/table_reader_bench.cc.o  -o table_reader_bench -Wl,-rpath,/develop/src/rocksdb/build librocksdb.so.6.8.0 libtestharness.a /usr/lib/x86_64-linux-gnu/libgflags.so third-party/gtest-1.8.1/fused-src/gtest/libgtest.a -lpthread
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6572

Reviewed By: anand1976

Differential Revision: D20789059

Pulled By: ajkr

fbshipit-source-id: 97329f14b9044b12c8a415da3d5f27b256ff8ff7

third-party/gtest-1.8.1/fused-src/gtest/CMakeLists.txt

@@ -1 +1,4 @@
 add_library(gtest gtest-all.cc)
+
+# Add dependency of gtest on thread library
+target_link_libraries(gtest ${CMAKE_THREAD_LIBS_INIT})