- Fix the check for --enable-compiler-plugins not being included in
  the resulting StaticLibrary_iwyudummy.mk file.
- Disable warnings about unused const variables to reduce noise.

Change-Id: If54dfd55f46631bbc526326bed5c272358fb9cf3
Reviewed-on: https://gerrit.libreoffice.org/24419Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>