make: tocklibrary: never build with -Werror

[bradjc]

Our build all file adds the -Werror CFLAG, which in general is good, but when compiling a library we don't necessarily have control over the library not having any warnings. This makes CI fail when using external libraries.

Todo

But, we want to enforce no warnings on libtock. Hmm...

[lschuermann]

Can't we include these libraries using -Isystem? I believe that should stop the compiler from reporting warnings for them.

[bradjc]

But in this case we are compiling the library in CI, not just using it.

[ppannuto]

I suspect if we added a $$((LIBNAME)_CPPFLAGS), which makes sense for other reasons probably, and added that after all the other flags here https://github.com/tock/libtock-c/blob/master/TockLibrary.mk#L102-L109, then you could add -Wno-error to those library flags. IIRC, last-flag has precedence in gcc when there is otherwise conflicts.