configure: Do not build libfdt if not required

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: Thomas Huth <thuth@redhat.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200118140619.26333-2-philmd@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>