Show a message when clang-tidy has been disabled.
Enable POLYSQUARE_USE_IWYU by default. Use correct function name.
Pass CPP_IDENTIFIERS, INTERNAL_EXCLUDE_DIRS and EXTERNAL_INCLUDE_DIRS
to the cmake tools.

Upgrade clang-tidy-target-cmake.
Upgrade veracpp-cmake. Adapt for new API.
Function with cppcheck versions before and after 1.58.

These versions of cppcheck either don't or do require a language
to be passed to them when scanning headers.