(no) over-writing generated files

[John Levon]

AC_CONFIG_HEADER() is neat - it checks for changes in config.h before
over-writing with the new version. This avoids unnecessary re-compiles
after a ./configure

Alas, AC_OUTPUT() is not so smart: the file is always over-written. With
our "version.h" header, it always gets over-written, causing spurious
re-compiles.

Would it be possible to have the same behaviour for AC_OUTPUT() files ?

Alternatively, there could be a macro that does the necessary
substitution (e.g. of @PACKAGE@ and @VERSION@) in the given files, and
still uses the config.h trick above

I'm using autoconf 2.56 and automake 1.7.1, but our baseline is
autoconf 2.13 and automake 1.5

thanks
john
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