> Say I have a script that reads /proc/cpuinfo and runs my executable with the
> correct flags to load the library with the best CPU features possible. How can
> I embed such a script in the package definition (as a gexp?) and install it
> under <out>/bin/?

Let's presume the binary is called $X.

What I would do: add a build phase after the "install" phase that renames
<out>/bin/$X to <out>/bin/.$X-real using the rename-file procedure.  Create
your wrapper script at <out>/bin/.$X-real with call-with-output-file, some
I/O procedures and chmod (to make the wrapper script executable).

I hope that helps, Maxime.