How can I keep CXXFLAGS in sync in all the directories in a shallow pacakge?

[Eric Hanchrow]

I like to pass certain flags, such as `-Wall', to the C++ compiler in
all the directories of my package.  Originally I had `CXXFLAGS=-Wall'
in each of the Makefile.ams, but that duplication seemed wrong, and I
sought a cleaner way to do it.  I haven't yet found one.

Here's how I'm now doing it, which is also unsatisfying:

I'm not setting CXXFLAGS at all, and instead have the top-level
Makefile.am look like this:

    AM_CXXFLAGS=-Wall
    export AM_CXXFLAGS

This seems to work, but the automake manual says 

        Using [GNU make] extensions in a `Makefile.am' will lead to
        errors or confusing behavior.

... and I *think* that `export' is a GNU make extension.  So I'm still
looking for a better solution.  (Right now I'm using an autoconf hack
(http://www.gnu.org/software/ac-archive/Installed_Packages/check_gnu_make.html))
to require GNU make, but that's ugly.)

Even if it turns out to be safe to use `export', I'm still wondering
if this is the best way to keep CXXFLAGS the same in all directories.
Any ideas?

Please respond directly to me, as I don't read this list.

Thanks!
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