Fixed finding vala with version in filenames

Build failed on systems when version was appended to valac-.

The problem was solved by using the second and more complete definition of the find_program command.
http://cmake.org/cmake/help/v2.8.8/cmake.html#command:find_program
Merge pull request #111 from placidrage/master

Fixed finding valac with version in filename. Thanks to Joe R. Nassimian!