Update gdal.rb

.include? 'something' is returning false in the configure options on OS X w/ Ruby 2.0.0-p0 (other versions not tested) while throwing (in IRB):

```
warning: string literal in condition
```

Example output of the error this fixes:

```bash
==> ./configure --prefix=/usr/local/Cellar/gdal/1.9.2 --mandir=/usr/local/Cellar/gdal/1.9.2/share/man --with-local=/usr/local/Cellar/gdal/1.9.2 -
configure: WARNING: you should use --build, --host, --target
configure: WARNING: you should use --build, --host, --target
configure: WARNING: you should use --build, --host, --target
checking build system type... Invalid configuration `false': machine `false' not recognized
configure: error: /bin/sh ./config.sub false failed

READ THIS: https://github.com/mxcl/homebrew/wiki/troubleshooting

These open issues may also help:
    https://github.com/mxcl/homebrew/issues/18262
```

Adding parens fixes the issue and lets the install work as intended.