Built sources and make distcheck

[Tavian Barnes]

I have a project which auto-generates some look-up tables when checked
out from its SCM, and are distributed in the tarballs.  These tables
used to be made by shell scripts called from autogen.sh, but I
recently ported them to C for a massive speed improvement.  I've
implemented the Makefile.am like this:

EXTRA_PROGRAMS = generate
generate_SOURCES = generate.c

generated.c: generate
        ./generate$(EXEEXT) >$@

bin_PROGRAMS = lookup
lookup_SOURCES = lookup.c generated.c

This is almost right, but because generated.c gets distributed but
`generate' doesn't, make sees generated.c as out-of-date in the
tarballs, and re-builds it.  This is worse when `make distcheck' is
run, because of the VPATH build; $@ isn't even the right location to
write to, and the source directory is read-only anyway, so a qualified
path wouldn't help.  Is there any way to not build the `generate'
target if generated.c exists, but still build it when it doesn't?

Thanks,
--
Tavian Barnes