multimedia/vlc: add ALTIVEC option, fix build on powerpc64 elfv2, add USES=localbase

Add ALTIVEC option to enable/disable AltiVec easily when needed.

Fix build with LLVM and AltiVec, altivec.h defines its own bool.

Add USES=localbase.

PR: 240537
Approved by: linimon (mentor), multimedia (maintainer timeout)