This fixes issue #239 related to building Ruby on FreeBSD systems

This relates to issue #239 that caused Ruby-build to fail when building Ruby on FreeBSD systems. 

I tracked down the error, and it was caused by running make with the `make -j 2`-parameter.  

Why it failed I don't know, but it would seem that using `-j` is disabling backwards compatibility, from the make man-page:

 > -B:
>             Try to be backwards compatible by executing a single shell per
 >            command and by executing the commands to make the sources of a
 >            dependency line in sequence.  This is turned on by default unless
 >            -j is used.

> -j max_jobs
>             Specify the maximum number of jobs that make may have running at
>             any one time.  Turns compatibility mode off, unless the -B flag
>             is also specified.


Before my change, running `rbenv install 1.9.3-p327` ran `make -j 2` would cause the compiling to fail:

```
installing default wait_for_single_fd libraries
compiling wait_for_single_fd.c
linking shared-object -test-/wait_for_single_fd/wait_for_single_fd.so
make: don't know how to make ../../.ext/common/bigdecimal. Stop
*** [ext/bigdecimal/all] Error code 2
1 error
*** [build-ext] Error code 2
```

After my change, the build would complete with `make -B -j 2`. 

I don't know if my solution is the "prettiest", but maybe we should consider just adding the -B argument on other non-freebsd systems as well.

Thoughts about this?