Set OPENMP to -fopenmp

Fix the error:
```
g++ -I.  lsb.o Kmer.o KmerIterator.o hash.o RepHash.o KmerStream.o  -lm -lz  -o KmerStream
Undefined symbols for architecture x86_64:
  "_GOMP_barrier", referenced from:
      void RunThreadedFastqStream<ReadQualityHasher>(ProgramOptions const&) [clone ._omp_fn.0] in KmerStream.o
```