rrdtool: fixes build with XCode 5.1

Currently, building rrdtool fails on my system (see config below).

According to me this is due to the new clang 'sensitivity to unused command line arguments' provided with Xcode 5.1.

This pull request provides the patch that allowed me to install rrdtool on my system.

Hollie

```
$ brew --config
HOMEBREW_VERSION: 0.9.5
ORIGIN: https://github.com/Homebrew/homebrew.git
HEAD: 44a716b154efd8130300e9abfe78fa03fe8c5166
HOMEBREW_PREFIX: /usr/local
HOMEBREW_CELLAR: /usr/local/Cellar
CPU: quad-core 64-bit sandybridge
OS X: 10.9.2-x86_64
Xcode: 5.1.1
CLT: 5.1.0.0.1.1396320587
Clang: 5.1 build 503
X11: 2.7.4 => /opt/X11
System Ruby: 1.8.7-358
Perl: /Users/lieven/perl5/perlbrew/perls/perl-5.18.2/bin/perl
Python: /usr/bin/python
Ruby: /usr/bin/ruby => /System/Library/Frameworks/Ruby.framework/Versions/2.0/usr/bin/ruby
```