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 #373972
mingw-build

00:06:28

passed #373970
shared-build

00:05:10

passed #373971
static-build

00:04:00