Add support for disabling generation of executables.

The `optcomp` package is a dependency of the Mirage unikernel [1] stack, and I am working on a new backend for it.  However, this backend uses a specialized cross-compiler that -- due to certain constraints -- cannot build the `optcomp` binaries so I would need an option to disable that.  For what it is worth, those binaries are not needed, as only the library is utilized in Mirage, hence I added some support for disabling building binaries via an environment variable, `NO_EXECUTABLES`.  Albeit, I believe the conditional compilation of binaries might be useful for others too.

[1] http://openmirage.org/