[Polly][CMake] Exclude isl_config from the polly-check-format target.

Summary:
The custom `polly-check-format` target runs clang-format over all source files in the directory tree excluding lib/External. `isl_config.h` is a header file that is generated by CMake in the build directory, and it's not correctly formatted (which I also wouldn't consider necessary, as it is a generated file).

If the build directory is actually inside the Polly source directory (which it might be if you're building Polly out-of-tree), that check always fails. Hence this patch excludes this file from the check-format target.

Reviewers: Meinersbur, grosser

Reviewed By: grosser

Subscribers: mgorny, llvm-commits, pollydev

Tags: #polly

Differential Revision: https://reviews.llvm.org/D33192

llvm-svn: 303060