also include libgomp.a in list of libraries for multithreading for GCC

Bug fix, this is required when building HPL with a gimkl toolchain for example, to avoid errors like:

```
/path/to/software/imkl/11.1.2.144-GCC-4.8.3/mkl/lib/intel64/libmkl_gnu_thread.so:
undefined reference to `GOMP_critical_end'
/path/to/software/imkl/11.1.2.144-GCC-4.8.3/mkl/lib/intel64/libmkl_gnu_thread.so:
undefined reference to `omp_get_thread_num'
```

see also https://software.intel.com/en-us/forums/topic/501970 (different problem there, but related)