Let bcc debian build depend on 3.7|3.8
Let bcc debian build depend on 3.7|3.8

In Xenial, llvm 3.8 will be available upstream without manual apt steps.
Add those packages as a possible dependency in our build.

brendangregg/bcc,brendangregg/bcc,iovisor/bcc,tuxology/bcc,romain-intel/bcc,shodoco/bcc,romain-intel/bcc,shodoco/bcc,shodoco/bcc,mkacik/bcc,tuxology/bcc,brendangregg/bcc,mkacik/bcc,tuxology/bcc,mkacik/bcc,iovisor/bcc,zaafar/bcc,mcaleavya/bcc,mcaleavya/bcc,brendangregg/bcc,zaafar/bcc,iovisor/bcc,mcaleavya/bcc,shodoco/bcc,mcaleavya/bcc,romain-intel/bcc,zaafar/bcc,mkacik/bcc,zaafar/bcc,tuxology/bcc,tuxology/bcc,shodoco/bcc,mcaleavya/bcc,brendangregg/bcc,romain-intel/bcc,iovisor/bcc,mkacik/bcc,zaafar/bcc,iovisor/bcc,romain-intel/bcc