Re-add the autoconf rule for the docs/doxygen.cfg file.
For some reason this was not reverted when r103213 was.

At the same time, add an optional rule for clang's doxygen.cfg.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@131279 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed