Installing v6.0.0-rc.1 does not populate /usr/local/include

# Issue

I have a downstream project using libosrm, relying on `pkg-config` to link to the relevant header files. Those have historically always been installed in `/usr/local/include/osrm`.

But when following the usual build + install steps with the latest tag `v6.0.0-rc.1`, no folder `/usr/local/include/osrm` is created.

This is what I used to get after build + install with a previous dev version (`master` at d25984845):

```
$ ls /usr/local/include/osrm/
approach.hpp           contractor.hpp         datasets.hpp       exception.hpp         json_container.hpp      osrm.hpp                route_parameters.hpp  table_parameters.hpp
bearing.hpp            coordinate.hpp         engine             extractor             match_parameters.hpp    partitioner             status.hpp            tile_parameters.hpp
contractor             customizer_config.hpp  engine_config.hpp  extractor_config.hpp  nearest_parameters.hpp  partitioner_config.hpp  storage               trip_parameters.hpp
contractor_config.hpp  customizer.hpp         error_codes.hpp    extractor.hpp         osrm_fwd.hpp            partitioner.hpp         storage_config.hpp    util
```

Link to a downstream CI check containing all OSRM build and install steps, then failing to find a libosrm header file: https://github.com/VROOM-Project/vroom/actions/runs/13973127966/job/39119910020?pr=1235

# Specifications

Build/install done on Ubuntu 24.04 with gcc.

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