Add polly support to the build system.

Add polly support to the build system.

Update the cmake and autoconf build system to compile polly
as a shared library if it is checked out into tools/polly. In case
polly is not checked out, nothing changes.
This models the way clang can be added to llvm if checked out to tools/clang.

Also rebuild configure.

Patch contributed by ether.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@117755 91177308-0d34-0410-b5e6-96231b3b80d8