CGALConfig.cmake: resolve symlinks before computing CGAL_CONFIG_DIR (fix #8521) (#9390)

## Summary of Changes

On distributions such as Arch Linux and Fedora where `/lib` is a symlink
to `/usr/lib`, `CMAKE_CURRENT_LIST_FILE` may resolve to
`/lib/cmake/CGAL/CGALConfig.cmake`. The subsequent chain of
`get_filename_component(... DIRECTORY)` calls then walks up through the
symlink path, producing `CGAL_ROOT=/` instead of `/usr`, and the
existence check for `include/CGAL/config.h` fails — even though CGAL is
properly installed.

**Fix:** resolve the real path of `CMAKE_CURRENT_LIST_FILE` before
extracting the directory. Uses `file(REAL_PATH)` when available (CMake
>= 3.19), with a `get_filename_component(... REALPATH)` fallback for
CMake 3.15–3.18.

Reproducer (from @lrineau's comment):
```
CGAL_DIR=/lib/cmake/CGAL cmake . -B build
# → CGAL_FOUND=FALSE, even though CGAL is installed
```

## Release Management

* Affected package(s): Installation
* Issue(s) solved (if any): fix #8521
* License and copyright ownership: same as CGAL

Author: sloriot

Installation/lib/cmake/CGAL/CGALConfig.cmake

@@ -2,12 +2,18 @@
 # This file is the CGALConfig.cmake for a header-only CGAL installation
 #
 
+cmake_minimum_required(VERSION 3.22...3.31)
+
 # For CGAL_CreateSingleSourceCGALProgram.cmake
 set( CGAL_REQUESTED_COMPONENTS ${CGAL_FIND_COMPONENTS} )
 
 set(CGAL_LIBRARIES CGAL)
 
-get_filename_component(CGAL_CONFIG_DIR "${CMAKE_CURRENT_LIST_FILE}" PATH)
+# Resolve symlinks before extracting the directory so that the subsequent
+# DIRECTORY walk produces a correct CGAL_ROOT (e.g. /lib -> /usr/lib on
+# Arch Linux).  See https://github.com/CGAL/cgal/issues/8521
+file(REAL_PATH "${CMAKE_CURRENT_LIST_FILE}" _cgal_config_realpath)
+get_filename_component(CGAL_CONFIG_DIR "${_cgal_config_realpath}" DIRECTORY)
 
 function(cgal_detect_branch_build VAR_NAME)
   if(IS_DIRECTORY ${CGAL_CONFIG_DIR}/../../../../Installation/package_info/Installation/)