Use `-Werror=undef` when compiling bcc to catch issues when a macro is not defined.

[chantra]

A macro which is not defined will default to 0, this can make it challenging to debug issues as the compiler will happily go ahead and treat an undefined macro to 0.

Here is an example case where it caught an issue: https://github.com/bpftrace/bpftrace/pull/2950/commits/a0eefb6491431d90457c38107a7724f4303e8312

[chantra]

cc @yonghong-song as we were discussing this f2f.