Can't install on Haiku R1 beta 4

mimalloc builds just fine, but it appears that there's something wrong with the default install path. Or am I doing something wrong?

```
~/mimalloc/build> make install
[ 40%] Built target mimalloc
[ 81%] Built target mimalloc-static
[ 83%] Built target mimalloc-obj
[ 89%] Built target mimalloc-test-api
[ 94%] Built target mimalloc-test-api-fill
[100%] Built target mimalloc-test-stress
Install the project...
-- Install configuration: "Release"
-- Installing: /boot/system/lib/libmimalloc.so.2.0
CMake Error at cmake_install.cmake:52 (file):
file INSTALL cannot copy file
"/boot/home/mimalloc/build/libmimalloc.so.2.0" to
"/boot/system/lib/libmimalloc.so.2.0": Read-only file system.
```

Start with the CMake install configuration that produces cmake_install.cmake and the reported destination /boot/system/lib/libmimalloc.so.2.0. Check how installation paths should work on Haiku R1 beta 4, then reproduce the failure with make install and verify that the corrected behavior no longer attempts to write to the read-only path.

Written by the indexing model from the issue text.