WITH_FLAC option is a no-op

ignoring docbook and packaging subdir, grep returns only this:

CMakeLists.txt:  set(WITH_FLAC OFF CACHE BOOL "build with FLAC")
CMakeLists.txt:  set(WITH_FLAC ON CACHE BOOL "build with FLAC")
INSTALL:WITH_FLAC: build with FLAC

This is a problem when actually being relied upon in src/plugins/oggflacmetadata, without flac being installed:

CMake Error at src/plugins/oggflacmetadata/CMakeLists.txt:31 (target_link_libraries):
  Target "oggflacmetadata" links to:

    FLACPP::FLACPP

  but the target was not found.

The CMakeLists.txt inside that plugin dir also pretends to consider HAVE_FLAC, but then goes on to unconditionally use FLACPP::FLACPP.

Seems to be a regression from commit 365af1eb.