Does not compile with Clang

Relevant for IKOS.

The cobra package's makefile has [unsupported arguments for clang compilers](https://github.com/nimble-code/Cobra/blob/master/src/makefile#L32), which causes build errors,

```
make[3]: warning: jobserver unavailable: using -j1. Add '+' to parent make rule.
error: unknown warning option '-Wformat-truncation=1' [-Werror,-Wunknown-warning-option]
make[3]: *** [: cobra_prim.o] Error 1
gmake[2]: *** [CMakeFiles/cobra-4.8.dir/build.make:86: cobra-4.8/src/cobra-4.8-stamp/cobra-4.8-build] Error 2
gmake[1]: *** [CMakeFiles/Makefile2:137: CMakeFiles/cobra-4.8.dir/all] Error 2
gmake: *** [Makefile:134: all] Error 2
--- stderr: cobra_vendor
Cloning into 'cobra-4.8'...
HEAD is now at 5af7230 Add files via upload
make[3]: warning: jobserver unavailable: using -j1. Add '+' to parent make rule.
error: unknown warning option '-Wformat-truncation=1' [-Werror,-Wunknown-warning-option]
make[3]: *** [: cobra_prim.o] Error 1
gmake[2]: *** [CMakeFiles/cobra-4.8.dir/build.make:86: cobra-4.8/src/cobra-4.8-stamp/cobra-4.8-build] Error 2
gmake[1]: *** [CMakeFiles/Makefile2:137: CMakeFiles/cobra-4.8.dir/all] Error 2
gmake: *** [Makefile:134: all] Error 2
---
Failed <<< cobra_vendor [3.25s, exited with code 2]
```

This prevents the ikos scanning tool from running on the package.