llvm-config: Add --build-system option

Summary:
This can be used for distinguishing between cmake and autoconf builds.
Users may need this in order to handle inconsistencies between the
outputs of the two build systems.

Reviewers: echristo, chandlerc, beanz

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11838

llvm-svn: 247159