[xml] [Bugzilla #327515] distribution broken for builds where builddir != srcdir

[Tim Van Holder <tim vanholder anubex com>]

The tarball for libxslt 1.1.15 ships with libxslt/xsltconfig.h (and
libxslt/xsltwin32config.h) included for the benefit of Windows builds
that do not use configure (mainly msvc).
However, if the tree is configured & built outside the source tree,
these take precedence over the ones created by configure, breaking
the build (e.g. it will always try to use modules, even if libxml2
was correctly detected as not having module support).

Possible solutions:

1. try to change #include precedence so that builddir comes first
   => cons: might not be possible (might be the autotools that
            handle this)
2. erase the headers from srcdir at configure time
   => cons: breaks for read-only srcdir (not uncommon)
3. rename the distributed headers to .w32, and have configure.js
   copy them to .h before starting a non-configure Windows build
   => cons: not really
4. place the headers in the win32 subdir, and have configure.js
   copy them before starting a non-configure Windows build
   => cons: not really
5. place the headers in the win32 subdir, and use -I$top_srcdir/win32
   to find them
   => cons: requires maintaining this option in each of the win32
            makefiles and projects

I'm personally not in favor of options 1 and 2; but I have no
preference when it comes to the others.
Some feedback would be welcome, so that Daniel Veillard can
resolve the bugzilla entry without angering the win32 user
base in the process.