Sam Yates authored May 31, 2018 and Brad King committed Jun 14, 2018

find_program() incorrectly prepended search path components
to absolute file paths, and incorrectly searched the current
working directory for files that contained no directory
separators.

* Replace calls cmFindProgramHelper::CheckDirectory(std::string())
  with call of new method cmFindProgramHelper::CheckCompoundNames()
  that checks for the presence of a directory separator in the
  file name.
* Use cmSystemTools::CollapseCombinedPath rather than string
  concatenation to properly combine absolute file names with
  search path components.
* Add unit tests to verify corrections.

Fixes: #18044