CMake executable detection does not account for directories correctly on UNIX

[eviltak]

As per the documentation of java.nio.file.Files.isExecutable, the method can return true when passed a directory on UNIX systems. This causes the Executables.isPathValid helper to incorrectly return true when given a directory named cmake.

Replication

• Create a directory called cmake under the current working directory
• Run CMakeInstance.generate() in the same working directory.
• The cmake directory will incorrectly be assumed to be a cmake executable, and CMakeInstance will attempt to execute it.

Potential fix

Add a Files.isDirectory disqualification check to the isPathValid helper.