We default system-integration to true as this is a shared library that
must be installed on a library path and it is recommended to use the
system package.
If the system does not package gtk4-layer-shell then doing `zig build
-fno-sys` will now correctly build and install the shared library under
a lib/ subdirectory of the output prefix.
The output prefix should be a default library path (`/lib`, `/usr/lib`,
or a lib64 variant) otherwise a custom library path can be configured
using ldconfig (see `man ld.so 8`)
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