Just attempted a similar review of the GNU site and could not determine if GNU/GCC really isn't at the ANSI-99 level, couldn't determine it with a simple search of the site.
It appears that there are non-free tools that do pure ANSI-C 99 development, and make it clear that they do. As to the no-cost stuff for Microsoft or GNU, there appears to be no firm answer that I can come up with in just a few minutes.
If anyone can determine the exact level of either GNU or Microsoft (I am pretty sure it's 1995), please add to this comment tree.
As far as I can tell the C-Language that Microsoft supports is not fully ANSI-C 1999 compliant. The best I can tell from the corporate documentation is that the C compiler is compliant up to ANSI-C 1995, but that is only my take from a brief Bing search
of the MSDN sites and Microsoft.com sites.
Even gcc is only compliant to the ANSI-C 95 level as far as I can tell, let me know otherwise.