AM_CPPFLAGS best practice

[Eric Blake]

The manual (gnulib-tool.texi) recommends including both builddir and
srcdir into AM_CPPFLAGS (aka INCLUDES if you are using automake 1.9),
which makes sense, since some headers come straight from gnulib and live
in srcdir, while other headers are generated based on configure results
and thus live in builddir.  But which order is preferred?

The gnulib manual recommends:
AM_CPPFLAGS = -I$(top_builddir)/lib -I$(top_srcdir)/lib

But m4 is using the reverse order:
AM_CPPFLAGS = -I$(top_srcdir)/lib -I../lib

And coreutils has a bug, for forgetting the builddir version (which
means it will build in an in-tree build, but probably fail in a VPATH
build):
AM_CPPFLAGS = -I$(top_srcdir)/lib

Is there any specific reason that gnulib recommends builddir (generated
files) before srcdir?  Consider the ramifications if a file changes
location.  If it goes from generated to in-place (moves from builddir to
srcdir, because it no longer depends on configure results), then
favoring builddir first means that in an incremental build where you
forget to run 'make clean' before updating gnulib, the stale generated
header will be left over in the build tree and be picked up in
preference to the new gnulib file.  Conversely, if a header goes from
in-place to generated, it will no longer live in srcdir, but only in
builddir, so it doesn't matter whether you favor builddir or srcdir.
Therefore, it seems to me that best practice would be to always favor
srcdir first, builddir second.

Thoughts, before I submit patches to coreutils and to either gnulib or m4?

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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