"#include" equivalent in spec files?

[Jonathan Reed <jdreed MIT EDU>]

Is there any way, in a spec file, to include another spec file  
snippet, similar to #include in C or source/eval in shell scripts?

My dilemma is this:

I maintain many spec files which need to build on different  
distributions (RHEL, Fedora, Suse, etc).  I have achieved this  
through a series of custom macros.  These macros, which determine the  
build platform and distribution, are the same across all the spec  
files.  I find myself needing to update those macros from time to  
time, and would like to avoid changing every single spec file which  
contains them.

I can get around this by having a comment act as a placeholder in my  
spec file, and having my build system import the platform detection  
macros, but I'm hoping there's a way to do it entirely within  
rpmbuild and the spec file.  I played around with something like % 
(cat /path/to/spec), which seems to correctly import the file, but % 
define and other macros don't get processed, so that would only be  
useful for tags.

Thanks,

-Jon