Improve autoconf and automake files.
Use autoconf header.
Restore plugin functionality.
Restore f12automaat plugin functionality.
Improve autoconf and automake files.

Merge commit '7c28accc64731f3b831823a160a182194e436c4c' into dev
Merge commit '9c6bae6abe3d1709f80ff3bd0dc06b64fe7fbae9' into dev
Restore plugin functionality.

Merge commit 'e3ad4aaf56a7092aa57fab26695bc8e88fe07865' into dev
Restore f12automaat plugin functionality.

Merge commit '77f8c18309831e0946b16e8dc9eae2f4ae5aa865' into dev