Fails to build if xmllint is unavailable

Nix fails to build if xmllint is unavailable, but `configure` does not complain about missing xmllint. It just sets it to `false`:

```
configure.ac:154:AC_PATH_PROG(xmllint, xmllint, false)
```

```
# make V=1
false --nonet --xinclude doc/manual/manual.xml -o doc/manual/manual.xmli.tmp
doc/manual/local.mk:16: recipe for target 'doc/manual/manual.xmli' failed
```
