Apply correct cflags to source files

Fixes the last warning in gyp build:

```
  CC(target) Release/obj.target/binding/src/libsass/cencode.o
cc1: warning: command line option '-std=c++11' is valid for C++/ObjC++ but not for C [enabled by default]
cc1: warning: command line option '-std=c++11' is valid for C++/ObjC++ but not for C [enabled by default]
```

Changes flag to `-std=c++0x` since it is enough to compile and is supported by much earlier gcc versions!