[CMake] Add PACKAGE_VENDOR for customizing version output

Summary: This change adds a PACKAGE_VENDOR variable. When set it makes the version output more closely resemble the clang version output.

Reviewers: aprantl, bogner

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D18159

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@263566 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed