two ways of building a target, depending on different prerequisites

[José Romildo Malaquias]

Hello.

How can I write a make rule to build a target that can be built in two
different ways, depending on which pre-requisite files are available?

For instance, suppose that the target file test.txt can be built in two
different ways, according to the following two rules:

test.txt: file1.txt
          command1 $< $@

test.txt: file2.txt
          command2 $< $@

If file1.txt exists (or can be built), then the first rule is
used. Otherwise the second rule is used.

Romildo