meson.build: Fix the check for a usable libfdt

meson.build: Fix the check for a usable libfdt

The check for libfdt currently has a flaw: If there is a system libfdt, the
meson.build code initialized the fdt variable with fdt = cc.find_library(...).
However, if this libfdt is too old and there is no internal dtc module
available, it continues with "fdt" pointing to the old and unusable version.
The check later in the file that tries to detect whether libfdt is necessary
then fails to trigger:

 if not fdt.found() and fdt_required.length() > 0
  error('fdt not available but required by targets ' + ', '.join(fdt_required))
 endif

The build fails then during compilation instead, which is of course bad
since this is quite confusing and already wasted quite some time of the user.
Thus if libfdt is not usable, we should unset the "fdt" variable immediately
again, so that the build already fails during the configuration phase.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/255
Message-Id: <20210827120901.150276-2-thuth@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>