configure: Do not build libfdt if not required

We only require libfdt for system emulation, in a small set
of architecture:

4077  # fdt support is mandatory for at least some target architectures,
4078  # so insist on it if we're building those system emulators.
4079  fdt_required=no
4080  for target in $target_list; do
4081    case $target in
4082      aarch64*-softmmu|arm*-softmmu|ppc*-softmmu|microblaze*-softmmu|mips64el-softmmu|riscv*-softmmu)
4083        fdt_required=yes

Do not build libfdt if we did not manually specified --enable-fdt,
or have one of the platforms that require it in our target list.
Reviewed-by: NThomas Huth <thuth@redhat.com>
Reviewed-by: NAlistair Francis <alistair.francis@wdc.com>
Signed-off-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: NAlex Bennée <alex.bennee@linaro.org>
Message-Id: <20200118140619.26333-2-philmd@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

configure

@@ -4095,6 +4095,8 @@ if test "$fdt_required" = "yes"; then
       "targets which need it (by specifying a cut down --target-list)."
   fi
   fdt=yes
+elif test "$fdt" != "yes" ; then
+  fdt=no
 fi
 
 if test "$fdt" != "no" ; then