Only clean generated sources when in maintainer mode.

BUILT_SOURCES is only defined in maintainer mode. Use it in DISTCLEANFILES.

This fixes Debian #902362.

Signed-off-by: Mark Hindley <mark@hindley.org.uk>
2 jobs for master in 1 minute and 59 seconds
Status Job ID Name Coverage
  Build
passed #2715
build

00:00:51

 
  Distcheck
passed #2716
distcheck

00:01:08