system dependent extensions

[Daniel Robey]

This might just be a beginner's problem, but I can't find anything like
it in any documentation, so maybe you could help.  My code uses
system-dependent extensions for files (specifically matlab mex files)
I've made it so autoconf determines which extension to use, and I have
suffix rules set up. It stores it in the variable MEX_END.  I want to be
able to use that variable in my 'Makefile.am's to do something like
foo_$(MEX_END)_SOURCES = foo.c bar.c .... etc.   everything I try ends
up producing a message about bad macro names.  Is there an easier way?
Thanks for any help.