Make install places files in invalid paths

After cloning the repo, building and running make install the project is not properly installed.

Commands:
```
cd ./mimalloc
mkdir -p out/release
cd out/release
cmake ../../
make
make install
```

Resulting installation:
```
root@153362c728c0:/usr/local/lib# ls -R mimalloc-1.6/
mimalloc-1.6/:
cmake include libmimalloc.a libmimalloc.so libmimalloc.so.1.6 mimalloc.o

mimalloc-1.6/cmake:
mimalloc-config-version.cmake mimalloc-config.cmake mimalloc-release.cmake mimalloc.cmake

mimalloc-1.6/include:
mimalloc-new-delete.h mimalloc-override.h mimalloc.h
```

Reproduce the reported sequence in the issue: configure with cmake ../../, run make, then make install. Start by inspecting the CMake installation configuration and the resulting /usr/local/lib/mimalloc-1.6 tree. Done means installation no longer places the project files in invalid paths, with the intended layout clarified from the project configuration.

Written by the indexing model from the issue text.