html-xml-utils regressed cross building, because it now correctly
rebuilds and runs cexport in the native build. Unfortunately, it builds
it as a host tool, which means that it cannot be run during cross
compilation. Before the NMU, it wasn't run at all.

I'm attaching a patch to correctly rebuild cexport. Note that
AX_CC_FOR_BUILD is defined in the autoconf-archive package. While at it,
I'm also improving support for out-of-tree builds in the Makefile.

Helmut