Hi David,
After trying various options, the best solution seems to be the inclusion of target-specific Makefile fragments. Directly adding the rules to the Makefiles conflicts with Automake conditionals syntax. And Makefile fragments are easily extendible to other targets, such as Darwin. It avoids regenerating Makefile.in or configure for each change.
So, at the moment, this is a no-op for everything but AIX, but some other architectures might want to make use of it. Looks like a good approach to me, with just one request: Could you add a comment to Makefile.am what the line include $(srcdir)/empty.mk $(tmake_file) does? libfortran changes are OK. Regards Thomas