LLD Build Failure

error.txt

[1558/1647] Linking CXX shared library lib/liblldReaderWriter.so
FAILED: : && /usr/local/bin/clang++  -fPIC -fPIC -fvisibility-inlines-hidden -Wall -W -Wno-unused-parameter -Wwrite-strings -Wcast-qual -Wmissing-field-initializers -pedantic -Wno-long-long -Wcovered-switch-default -Wnon-virtual-dtor -Wdelete-non-virtual-dtor -std=c++11 -fcolor-diagnostics -ffunction-sections -fdata-sections -O3 -DNDEBUG  -Wl,-z,defs   -Wl,-O3 -Wl,--gc-sections -shared -Wl,-soname,liblldReaderWriter.so -o lib/liblldReaderWriter.so tools/lld/lib/ReaderWriter/CMakeFiles/lldReaderWriter.dir/CoreLinkingContext.cpp.o tools/lld/lib/ReaderWriter/CMakeFiles/lldReaderWriter.dir/FileArchive.cpp.o tools/lld/lib/ReaderWriter/CMakeFiles/lldReaderWriter.dir/LinkerScript.cpp.o  lib/liblldCore.so lib/liblldYAML.so lib/libLLVMObject.so lib/libLLVMSupport.so -Wl,-rpath,"\$ORIGIN/../lib" && :
tools/lld/lib/ReaderWriter/CMakeFiles/lldReaderWriter.dir/FileArchive.cpp.o:../../tools/lld/lib/ReaderWriter/FileArchive.cpp:function std::thread::thread<lld::internal::ThreadPoolExecutor::ThreadPoolExecutor(unsigned int)::{lambda()#1}>(lld::internal::ThreadPoolExecutor::ThreadPoolExecutor(unsigned int)::{lambda()#1}&&): error: undefined reference to 'pthread_create'
tools/lld/lib/ReaderWriter/CMakeFiles/lldReaderWriter.dir/FileArchive.cpp.o:../../tools/lld/lib/ReaderWriter/FileArchive.cpp:function std::thread::thread<lld::internal::ThreadPoolExecutor::ThreadPoolExecutor(unsigned int)::{lambda()#1}::operator()() const::{lambda()#1}>(lld::internal::ThreadPoolExecutor::ThreadPoolExecutor(unsigned int)::{lambda()#1}::operator()() const::{lambda()#1}&&): error: undefined reference to 'pthread_create'
clang-3.8: error: linker command failed with exit code 1 (use -v to see invocation)