spec: Add binpkg_blacklist option for troublesome packages

spec: Add binpkg_blacklist option for troublesome packages

For packages that don't use EAPI-5's sub-slots to track ABI
dependencies, binary packages may go stale without Portage noticing.
You can use this new option to list packages that may be troublesome.
Listed packages will be removed from the pkgcache, and rebuilt from
scratch for your seed.

If binpkg_blacklist is unset in your spec file, it defaults to
including the following packages:

* dev-libs/gcc: see, for example,
  http://mid.gmane.org/20130412011158.GB17116@odin.tremily.us
* media-libs/mesa: see, for example, #454184