configure: Display if libfdt is from system or git

The configure script outputs "yes" regardless which libfdt is used:

  ./configure
  [...]
  fdt support       yes

Sometimes you can have both system and local git version available,
change the configure script to display which library got selected:

  debian8$ dpkg-query --showformat='${Version}\n' --show libfdt-dev
  1.4.0+dfsg-1

  debian8$ ./configure
  [...]
  fdt support       git

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20180415230522.24404-3-f4bug@amsat.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>