fix fortran compiler detection on FreeBSD

On FreeBSD, passing extra options to `which` causes it to report a non-zero status:

```
$ which gfortran48 -m64
/usr/local/bin/gfortran48
$ echo $?
1
```

```
$ which gfortran48
/usr/local/bin/gfortran48
$ echo $?
0
```
Merge pull request #470 from nolta/patch-3

fix fortran compiler detection on FreeBSD