build: fix ICU support build on SmartOS

I've left the:
```
'cflags_c': ['-std=c99'],
```
changes for all supported platforms, not just for Solaris. 

This PR has been successfully built and tested on all supported platforms. Results are available for [Linux, MacOS X, SmartOS](http://jenkins.nodejs.org/job/nodejs-julien/) and [Windows](http://jenkins.nodejs.org/job/nodejs-julien-windows/).