move linker flags to the end

See: http://stackoverflow.com/a/1665110/330558

Before the patch, I get:

```
g++ -o lighter -Wall -O -lpthread ErrorCorrection.o KmerCode.o GetKmers.o main.o 
main.o: In function `main':
main.cpp:(.text+0x127f): undefined reference to `pthread_create'
main.cpp:(.text+0x129f): undefined reference to `pthread_join'
main.cpp:(.text+0x1923): undefined reference to `pthread_create'
main.cpp:(.text+0x1943): undefined reference to `pthread_join'
collect2: ld returned 1 exit status
make: *** [lighter] Error 1
```

After the patch, I can `make` successfully.