meson: Disable libdw for static builds by default

meson: Disable libdw for static builds by default

Static QEMU build fails on Debian Bullseye:

    /usr/bin/ld: /usr/lib/x86_64-linux-gnu/libdw.a(debuginfod-client.o): in function `__libdwfl_debuginfod_init':
    (.text.startup+0x17): undefined reference to `dlopen'

The reason is that pkg-config does not suggest -ldl for libdw, and
adding --extra-ldflags="-ldl" resolves the issue. However, static
linking with libdw is an unclear topic:

* Linux perf does it.
* Debian's libdw-dev description says:

      Only link to the static version for special cases and when you
      don't need anything from the ebl backends.

* As the error message above indicates, -ldl is also needed for
  debuginfod support.

The functionality provided by libdw is needed for analyzing performance
of JITed code, which is mostly useful to developers and researchers.
Therefore, in order to avoid unpleasant surprises for people who don't
need this, simply disable libdw for static builds by default. It can
still be enabled explicitly if needed.

Reported-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Message-Id: <20230210005208.438142-2-iii@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>