`find_include_file` incorrectly accepts directories as include files

[Rac75116]

https://github.com/boostorg/wave/blob/develop/include/boost/wave/util/cpp_include_paths.hpp#L331
https://github.com/boostorg/wave/blob/develop/include/boost/wave/util/cpp_include_paths.hpp#L365

boost::wave::utils::include_paths::find_include_file validates a candidate include path using boost::filesystem::exists. Because exists also returns true for directories, subsequent code that expects a file may attempt to open a directory and encounter a runtime error. Replacing exists with boost::filesystem::is_regular_file would ensure only regular files are accepted.

[hkaiser]

Nice catch! Thanks!