no 'system' llibfdt (or too old), subprojects/dtc/ populated, ./configure --disable-download fails
Description of problem:
./configure ... --disable-download, with subprojects/ pre-populated, fails.
Steps to reproduce:
1. ensure libfdt/dtc files/libs/binaries are *not* found in system
2. have subprojects/dtc pre-populated
3. ./configure --target-list=riscv32-softmmu --prefix=/opt/riscv --enable-debug --without-default-features --without-default-devices --disable-download

configure fails with:
```
../meson.build:3171:13: ERROR: C shared or static library 'fdt' not found

A full log can be found at /home/too/vc/ext/qemu/build/meson-logs/meson-log.txt

ERROR: meson setup failed
```

If I outcomment the following lines in meson.build:
```
    #if get_option('wrap_mode') == 'nodownload'
    #  fdt_opt = 'system'
    #endif
```
Then the above command line works (with --disable-download)
Additional information:
The case is where one wants to ensure that configure does not try to access
network while doing its job. And in a system where dtc/libfdt is not available,
(or is too old, line in Centos/RHEL 7) one has dowloaded the files already in
subprojects/dtc/.

The meson.build clearly sets (as of 2024-03-05) expectation that dtc/libfdt/
has to come from 'system' if 'wrap_mode' is set to 'nodownload'.

Without this check it it works nicely -- and if subprojects/dtc/ was not populated,
the error message is 

```
Library fdt found: NO

../meson.build:3187:18: ERROR: Automatic wrap-based subproject downloading is disabled

A full log can be found at /home/too/vc/ext/qemu/build/meson-logs/meson-log.txt

ERROR: meson setup failed
```

So -- to me -- that looks like it could be a suitable solution to this problem.