Automake simply copies make rules into the generated Makefile verbatim. So as long as what you write is valid makefile syntax, it's OK.
What you have there is part of a valid implicit suffix rule in make. However, you're missing that you need to define .rc
as a valid suffix (the .o
is part of the built-in suffix list so you don't have to add it, unless you just want to be complete).
Add this to your Makefile.am as well:
.SUFFIXES: .rc
See http://www.gnu.org/software/make/manual/html_node/Suffix-Rules.html for more details.