[CMake] Dependencies for EXEC_PROGRAM

[Neal Meyer]

I've got a static library that contains generated code, today this is
done in a make file so I can check the dependencies before the PERL
script is called again during the build process.  I'm adding a
CustomBuildCommand to the project, so the dependecies are properly
checked in my vcproj files.

I'm wondering if anybody has any clues on how this might be done in a
CMake file.  Right now I just have EXEC_PROGRAM being called
everytime, which just addes some time to the processing that seems to
be a waste.

-Neal