Makefile rule where one of the two prerequisites matches

I want to wite a makefile rule for a case, whe开发者_开发知识库re files of type .bbb and .ccc are compiled into .aaa.

I could simply write two rules:

%.aaa: %.bbb
   process $<
%.aaa: %.ccc
   process $<

but I'm wodering whether this could be accomplished in one rule, such as:

%.aaa: %.bbb OR %.ccc
   process $<

Note that only one of the files bbb or ccc is enough for compilation, and in my case, only one exists (so either x.bbb OR x.ccc, not both).

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The first way is how you express this. All makefile rules are implicitly ored together. Saying you can create %.aaa from either %.bbb or %.ccc is the same as saying you can create %.aaa from %.bbb and you can create %.aaa from %.ccc.

If the body of the rule is complex and you want to "reuse the body", many makes have macro facilities that will let you define it one place and reuse it.