Fix DSO missing from command line by adding lpthread to linker

Need to add -lpthread to resolve the following issues.

```
[ 97%] Building CXX object test/CMakeFiles/testeth.dir/trie.cpp.o
Linking CXX executable testeth
/usr/bin/ld: ../libethereum/libethereum.a(PeerServer.cpp.o): undefined reference to symbol 'pthread_create@@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
make[2]: *** [test/testeth] Error 1
make[1]: *** [test/CMakeFiles/testeth.dir/all] Error 2
make: *** [all] Error 2
```