It seems to have been accidentally enabled by the switch to making
dtrace a Meson feature. This has only just been caught because the
FreeBSD CI runner has been offline for several weeks (see
Infrastructure/Infrastructure#1503).

With dtrace enabled, the FreeBSD CI build fails with:
```
[8/1601] Generating 'gobject/libgobject-2.0.so.0.8100.0.p/gobject_probes.o'
FAILED: gobject/libgobject-2.0.so.0.8100.0.p/gobject_probes.o
/usr/sbin/dtrace -G -s ../gobject/gobject_probes.d -o gobject/libgobject-2.0.so.0.8100.0.p/gobject_probes.o
dtrace: failed to link script ../gobject/gobject_probes.d: No probe sites found for declared provider
[9/1601] Generating 'glib/libglib-2.0.so.0.8100.0.p/glib_probes.h' (wrapped by meson because command contains newlines)
[10/1601] Generating 'glib/libglib-2.0.so.0.8100.0.p/glib_probes.o'
FAILED: glib/libglib-2.0.so.0.8100.0.p/glib_probes.o
/usr/sbin/dtrace -G -s ../glib/glib_probes.d -o glib/libglib-2.0.s...