[CMake] Suppressing a compilation option for one file?

[Bryan O'Sullivan]

I know that I can use set_source_files_properties to add to the 
compilation flags used to build a single source file, but is there a way 
to *remove* a compilation flag for one file?

My problem is that I build the source tree with -Wall -Werror, but one 
source file contains a use of a bad macro from a third party package for 
which there's no gcc option to override the warning that gets generated, 
so I need to turn off -Werror instead.

	<b