Don't clean up pre-built docs if no docbook installed.

Fixes fontconfig/fontconfig#165
3 jobs for issue/165 in 6 minutes and 29 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed mingw-build #373972

00:06:28

passed shared-build #373970

00:05:10

passed static-build #373971

00:04:00