More robust OPENBLAS_ prefixing of macros in openblas_config.h

I cannot reproduce it but in the past I had a blank line in my `config_last.h`. The awk replacement of that line introduced to fix #315 was generating an invalid `openblas_config.h` header in that case.

The `NF` check should prevent this to happen again.

For reference, the error message I had with GCC was:

```
In file included from /opt/OpenBLAS/include/cblas.h:5:0,
                 from /home/ogrisel/tmp/my_file.c:33:
/opt/OpenBLAS/include/openblas_config.h:66:1: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘typedef’
```