RHEL 5-era autoconf did not define these, so define them manually
when they are missing.
Define builddir as '.' The scripts require this. It won't work
in the srcdir != builddir case, but we don't care about that for
RHEL 5.
This commit also moves the builddir / abs_srcdir variable setting
above the include of subdir-rules.mk, in case that include uses
these variables.
Useful script:
for f in $(find -name Makefile.am | xargs fgrep '$(abs_srcdir)' -l) ; do
if ! grep -q '^abs_srcdir' $f; then
echo missing in $f
fi
done