build: Use specific Meson options to enable debug flags
Based on gtk@eaef24c5
Note that TRACKER_DEBUG is disabled in most distro builds as they
pass --buildtype=plain
. I recommend distros add -DG_ENABLE_DEBUG
to Tracker Miners' CFLAGS as the overhead is minimal and it helps users
to provide useful bug reports.
See also: tracker!316 (merged)