Incorrect installation directory for pkg-config files.

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### Describe your issue here

The pkg-config scripts (`tools/pkg-config/*.pc`) are always installed in the wrong location. This led to all Linux distros trying different tricks to override the wrong installation.



This happens because `SFML_PKGCONFIG_DIR` is not being expanded correctly.

In `Config.cmake`:

```cmake

set(SFML_PKGCONFIG_DIR "/${CMAKE_INSTALL_LIBDIR}/pkgconfig")

```

After the **first** `cmake` run, this variable expands to

```

//pkgconfig

```

After the **second** `cmake` run, it changes to:

```

/lib64/pkgconfig

```

It appears that `CMAKE_INSTALL_LIBDIR` is set AFTER it's used to set `SFML_PKGCONFIG_DIR`.

I think the problem is caused by the wrong order of inclusion in `CMakeLists.txt`:



```cmake

# include the configuration file

include(cmake/Config.cmake)



# we use the paths from the cmake GNUInstallDirs module as defaults

# you can override these if you like

# https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html

include(GNUInstallDirs)

```



If I change it to include `GNUInstallDirs` before `cmake/Config.cmake`, the value seems to be set correctly.

### Your Environment

- OS / distro / window manager: Fedora 39, Mageia 9, Ubuntu Mantic, Debian Bookworm, Arch Linux (latest stable releases of all of them)

- SFML version: 2.6.1

- Compiler / toolchain: GCC

- Special compiler / CMake flags: none



### Steps to reproduce

1. Edit `cmake/Config.cmake`, appending these lines at the bottom:

```cmake

message(STATUS ":::: CMAKE_INSTALL_LIBDIR = ${CMAKE_INSTALL_LIBDIR}")

message(STATUS ":::: SFML_PKGCONFIG_DIR = ${SFML_PKGCONFIG_DIR}")

```

2. `mkdir build`

3. `cd build`

4. `cmake ..`

5. `cmake ..`

### Expected behavior

It should always print

```

-- :::: CMAKE_INSTALL_LIBDIR = lib64

-- :::: SFML_PKGCONFIG_DIR = /lib64/pkgconfig

```

each time `cmake` is run.

### Actual behavior

First `cmake` run prints:

```

-- :::: CMAKE_INSTALL_LIBDIR = 

-- :::: SFML_PKGCONFIG_DIR = //pkgconfig

```

The second run prints:

```

-- :::: CMAKE_INSTALL_LIBDIR = lib64

-- :::: SFML_PKGCONFIG_DIR = /lib64/pkgconfig

```

[]