environment variable conventions

[David Lee]

I am working on a project in which the end-user's configure would like to
ensure that certain items appear in autoconf output variables such as
CPPFLAGS.  We have received a query from a user whose routine "configure"
usage at his site not a simple "./configure" but rather is of the form:
    CPPFLAGS="..."  ./configure

How should the incoming CPPFLAGS (from the user's environment) normally
interact with any being set in "configure".  My guess is that configure's
CPPFLAGS should modify (prepend? append?) any setting from the environment
rather than simply replace it.

But presumably in the case of "CC", the operation would be a simple
replacement.  What about CFLAGS, LDFLAGS etc.?

Are these principles documented somewhere?  In neither the autoconf manual
nor the "GNU Autoconf, Automake and Libtool" book did I find anything but,
of course, I may well have missed it, so a pointer would be fine.

Thanks.

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