Conditionally use a compiler flag for only a particular library?

I am having an issue with using `-std=c++11` with libraries that have `.c` files. But it is required by another library.

```
 - /usr/local/lib/haxe/lib/linc_nanovg/git/linc/linc_nanovg.cpp
 - /usr/local/lib/haxe/lib/linc_nanovg/git/lib/nanovg/src/nanovg.c
Error: error: invalid argument '-std=c++11' not allowed with 'C'
```