`linux/CMakeLists.txt`: support offline/sandboxed builds via OBJECTBOX_PREBUILT_DIR

Is there an existing issue?

• I have searched existing issues

Use case

objectbox_flutter_libs downloads libobjectbox.so at CMake configure time via
FetchContent_Populate. This makes it impossible to build in any network-isolated
environment — Flatpak/Flathub sandboxes, NixOS, Gentoo, Guix, and any distro build
system that restricts network access during the build.

The usual cmake workaround — setting FETCHCONTENT_SOURCE_DIR_OBJECTBOX-DOWNLOAD —
has no effect here. FetchContent_Populate (the pre-3.14 deprecated API) internally
spawns a cmake subbuild: a separate cmake process. Cache variables set in the
parent do not propagate to that subbuild. This is a documented cmake limitation.

Proposed solution

Check a cmake variable OBJECTBOX_PREBUILT_DIR before calling FetchContent_Populate.
If the variable points to an already-extracted archive, skip the download entirely:

if(DEFINED OBJECTBOX_PREBUILT_DIR AND EXISTS "${OBJECTBOX_PREBUILT_DIR}/lib/libobjectbox.so")
    set(objectbox-download_SOURCE_DIR "${OBJECTBOX_PREBUILT_DIR}")
else()
    FetchContent_GetProperties(objectbox-download)
    if(NOT objectbox-download_POPULATED)
        FetchContent_Populate(objectbox-download)
    endif()
endif()

Distro packagers would pass:

-DOBJECTBOX_PREBUILT_DIR=/path/to/extracted/objectbox-c

Describe alternatives you've considered

Current workaround

We maintain a local patch in the Flatpak manifest for
ebalistyka-app that checks a hardcoded
relative path. It works but is fragile and app-specific.

Additional context

This patch proposes no breaking changes

Flathub has growing Flutter adoption. Every Flutter app using objectbox needs this
patch to pass Flathub CI (which runs flatpak-builder --sandbox with no network).
A one-line upstream fix lets all downstream packagers drop their local patches.

Start with linux/CMakeLists.txt and inspect the existing FetchContent_Populate flow for objectbox-download. Verify that an extracted directory passed through -DOBJECTBOX_PREBUILT_DIR is used without downloading, while the existing download path still works; done means sandboxed or network-isolated configuration can complete with the prebuilt library.