The MAKE_RT routine was added in IDL 7.0 to make it easier to distribute an application that runs on the IDL Virtual Machine. Given the location of a .sav file and an application name, MAKE_RT copies the .sav file and all the IDL Virtual Machine files necessary to run it into the output directory. It even makes an system-dependent launcher program i.e. an .exe on Windows, .app on Mac, and launch script on other Unix systems. But to make a cross-platform application, it seemed necessary to run the MAKE_RT multiple times, once on each platform you intended to support.

But this post on comp.lang.idl-pvwave by Paulo Penteado, provided the simple solution: just copy the contents of the various bin.* directories from IDL distributions on all the platforms you intend to support to one installation i.e. an IDL über-installation. For example, if you copy the bin.darwin.x86_64 directory from a 64-bit Mac-Intel distribution to your own IDL bin directory, you can now use the MACINT64 keyword to MAKE_RT. Set all the keywords corresponding to the binary directories you have and a distribution that runs on all those platforms can be created with one call of MAKE_RT. (See a followup post that lists a few issues with the process on specific platforms. A Mac über-installation seems like the best choice.)

Here is a wrapper for MAKE_RT that automatically discovers which binary directories are present in your distribution and automatically sets the appropriate platform keywords to MAKE_RT.