Fix building melbourne for MRI on Linux

I ran into an error building Rubinius using MRI 1.9, which I pinned down to an error building Melbourne:

```
$ ruby -v
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-linux]

$ uname -a
Linux lewis-MacBookAir 3.5.0-17-generic #28-Ubuntu SMP Tue Oct 9 19:31:23 UTC 2012 x86_64 x86_64 x86_64 GNU/Linux

$ rake extensions:melbourne_build
...
LDSHARED build/melbourne20.so
/usr/bin/ld: build/grammar19.o: relocation R_X86_64_32 against `.rodata.str1.1' can not be used when making a shared object; recompile with -fPIC
build/grammar19.o: could not read symbols: Bad value
...
```

The `-fPIC` flag was removed from the `CFLAGS` [here](https://github.com/rubinius/rubinius/commit/fc5c94c#L4L223), so I've added it back in by adding the new `Rubinius::BUILD_CONFIG[:system_cflags]` (which contains `-fPIC` on linux).