Noticed missing target directory dependency as a build failure in
`make --shuffle` mode (added in https://savannah.gnu.org/bugs/index.php?62100):
Making all in doc
make[1]: Entering directory '/build/heimdal/doc'
MAKEINFO hx509.info
hx509.texi:15: @include: could not find vars.texi
hx509.texi:31: warning: undefined flag: PACKAGE_VERSION
hx509.texi:186: warning: undefined flag: PACKAGE_VERSION
make[1]: *** [Makefile:622: hx509.info] Error 1 shuffle=1656683556
The change moves vars.texi to BUILT_SOURCES to guarantee it's
presence when .info files start the build.
254e40294e