Added clang-tidy-target-cmake support.

By default all checked targets will be checked with clang-tidy, the llvm-* and
google-* checks excluded.

Use CLANG_TIDY_ENABLE_CHECK and CLANG_TIDY_DISABLE_CHECKS on each checked
target to enable or disable particular checks.

POLYSQUARE_USE_CLANG_TIDY can be set to OFF to disable clang-tidy for a
particular build.