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
Changes in commit v3.4.0-rc1~124^2~1 (cmFindProgramCommand: Re-implement
search using more flexible approach, 2015-09-01) did not preserve the
behavior of looking for the given name with no search path at all.
Fix this and add a test case covering finding an absolute path with
no search directories.
When more than one value is given to the NAMES option this command by
default will consider one name at a time and search every directory for
it. Add a NAMES_PER_DIR option to tell this command to consider one
directory at a time and search for all names in it.
Previously this command was tested only implicitly as part of larger
tests. Add a RunCMake.find_program test to cover find_program cases
specifically and independently.
