qt4: Improve moc handling

• configure.ac: Disable pinentry-qt4 or exit if moc not found.
• m4/qt.m4: Look for and prefer moc-qt4.
• qt4/Makefile.am: Add moc files to nodist and clean them.

• Files generated by MOC vary over the used Qt versions (even inside a Major release). Distributing them leads to errors if a different qt version is installed.
  
  GnuPG-bug-id: 1784