How do I pipe the output of a shell command into a macro?

rpmbuild allows macros to be populated with the output of shell commands by encapsulating the command like so: '%define my_macro %(my_command)'. Shell scripters will no doubt notice the similarity to bash's own command substitution mechanism, $()

More about this, and other macro-related bits can be found on the RPM Macros page:
http://www.rpm.org/wiki/PackagerDocs/Macros