Add PTHREAD dependency

I added compiler flags for pthread. This fixes following error.
```
[5/7] cxxprogram: build/kvs_proxy.cpp.1.o -> build/kvs_proxy
/usr/bin/ld: kvs_proxy.cpp.1.o: undefined reference to symbol 'pthread_mutexattr_settype@@GLIBC_2.2.5'
//lib/x86_64-linux-gnu/libpthread.so.0: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
```