Compile ISL into its own library

Refactor all ISL-related cmake build instructions into its own 
CMakeLists.txt and build as a separate library.

This is useful to apply ISL-related build flags to ISL only and not to 
Polly's files. Also, it the separation of both projects becomes clearer.

Proposed name of the library is Polly_isl. It is not "isl" to avoid 
mix-up with potentially installed libisl.{a|so}.

Tested configurations:
- Windows with cmake 3.2
- Ubuntu with cmake 3.0.2 
- Ubuntu with cmake 3.0.2  BUILD_SHARED_LIBS on
- Ubuntu with cmake 2.8.12.2 (LLVM minimum version)
- Ubuntu out-of-LLVM-tree

Differential Revision: http://reviews.llvm.org/D12810

llvm-svn: 248484