improvements to iwyudummy target

- split awk script into separate file
- make awk work on older awk version
- create new target in main Makefile.in to generate
  iwyudummy Makefile so we don't have to manually uncomment
  stuff
- exclude /usr includes from the generated makefile
- disable unused macros warnings to reduce noise
- add some sanity checking - prevent using the generated
  makefile with compiler-plugins enabled
- add new target for generating iwyu Makefile so we don't
  need to edit the Makefile when using it

Change-Id: I4af8eb7d1aa5419e546acb9ef905a0fe623db57d
Reviewed-on: https://gerrit.libreoffice.org/12980
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>