Normalizes the filename specific to the build system/filesystem used. This routine is used when we report the information about the flag definition location. For instance, if your build resides at some location you do not want to expose in the usage output, you can trim it to show only relevant part. For example: normalize_filename("/my_company/some_long_path/src/project/file.cc") might produce "project/file.cc".
Declared in <absl/flags/usage_config.h>
std::function<std::string(std::string_view)> normalize_filename;