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