find_program() doesn't find not-readable programs

This issue was created automatically from an original Mantis Issue. Further discussion may take place here.

Say, I'm trying to do find_program(SUDO NAMES sudo) and it fails while sudo is actually available and working.

find_program() calls cmSystemTools::FindProgram(), which in turn uses SystemTools::FileExists(). FileExists() calls access(filename, R_OK) to determine if the file is useful. This is wrong, because a program may be not readable and still executable. This is, for example, the case for setuid programs, as many distributions set ---s--x--x mode for them.

Also access() checks real user's permission instead of effective one.

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