🐛 Ensure spdlog dependency can be found from mqt-core install (#1263)

## Description

This pull request updates the build system to improve how the `spdlog`
dependency is handled, fixing problems unfortunately only discovered
after the `v3.3.0` release yesterday.
This also directly prepares a corresponding patch release.

## Checklist:

<!---
This checklist serves as a reminder of a couple of things that ensure
your pull request will be merged swiftly.
-->

- [x] The pull request only contains commits that are focused and
relevant to this change.
- [x] I have added appropriate tests that cover the new/changed
functionality.
- [x] I have updated the documentation to reflect these changes.
- [x] I have added entries to the changelog for any noteworthy
additions, changes, fixes, or removals.
- [x] I have added migration instructions to the upgrade guide (if
needed).
- [x] The changes follow the project's style guidelines and introduce no
new warnings.
- [x] The changes are fully tested and pass the CI checks.
- [x] I have reviewed my own code changes.


<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
  * Resolved spdlog discovery issues when consuming mqt-core via CMake.
  * Ensured correct installation path for bundled spdlog CMake files.
  * Treated spdlog headers as system includes to reduce build warnings.
* Updated CMake config to explicitly locate spdlog during dependency
resolution.

* **Documentation**
* Added 3.3.1 release entry with details of the spdlog fix and updated
links.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: burgholzer <[email protected]>

Author: burgholzer

CHANGELOG.md

@@ -9,6 +9,12 @@ This project adheres to [Semantic Versioning], with the exception that minor rel
 
 ## [Unreleased]
 
+## [3.3.1] - 2025-10-14
+
+### Fixed
+
+- 🐛 Ensure `spdlog` dependency can be found from `mqt-core` install ([#1263]) ([**@burgholzer**])
+
 ## [3.3.0] - 2025-10-13
 
 _If you are upgrading: please see [`UPGRADING.md`](UPGRADING.md#330)._
@@ -192,7 +198,8 @@ _📚 Refer to the [GitHub Release Notes](https://github.com/munich-quantum-tool
 
 <!-- Version links -->
 
-[unreleased]: https://github.com/munich-quantum-toolkit/core/compare/v3.3.0...HEAD
+[unreleased]: https://github.com/munich-quantum-toolkit/core/compare/v3.3.1...HEAD
+[3.3.1]: https://github.com/munich-quantum-toolkit/core/releases/tag/v3.3.1
 [3.3.0]: https://github.com/munich-quantum-toolkit/core/releases/tag/v3.3.0
 [3.2.1]: https://github.com/munich-quantum-toolkit/core/releases/tag/v3.2.1
 [3.2.0]: https://github.com/munich-quantum-toolkit/core/releases/tag/v3.2.0
@@ -204,6 +211,7 @@ _📚 Refer to the [GitHub Release Notes](https://github.com/munich-quantum-tool
 
 <!-- PR links -->
 
+[#1263]: https://github.com/munich-quantum-toolkit/core/pull/1263
 [#1247]: https://github.com/munich-quantum-toolkit/core/pull/1247
 [#1246]: https://github.com/munich-quantum-toolkit/core/pull/1246
 [#1236]: https://github.com/munich-quantum-toolkit/core/pull/1236

cmake/ExternalDependencies.cmake

@@ -97,9 +97,28 @@ set(SPDLOG_VERSION
 set(SPDLOG_URL https://github.com/gabime/spdlog/archive/refs/tags/v${SPDLOG_VERSION}.tar.gz)
 # Add position independent code for spdlog, this is required for python bindings on linux
 set(SPDLOG_BUILD_PIC ON)
+set(SPDLOG_SYSTEM_INCLUDES
+    ON
+    CACHE INTERNAL "Treat the library headers like system headers")
 cmake_dependent_option(SPDLOG_INSTALL "Install spdlog library" ON "MQT_CORE_INSTALL" OFF)
 FetchContent_Declare(spdlog URL ${SPDLOG_URL} FIND_PACKAGE_ARGS ${SPDLOG_VERSION})
 list(APPEND FETCH_PACKAGES spdlog)
 
 # Make all declared dependencies available.
 FetchContent_MakeAvailable(${FETCH_PACKAGES})
+
+# Patch for spdlog cmake files to be installed in a common cmake directory
+if(SPDLOG_INSTALL)
+  include(GNUInstallDirs)
+  install(
+    CODE "
+    file(GLOB SPDLOG_CMAKE_FILES
+      \"\${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/cmake/spdlog/*\")
+    if(SPDLOG_CMAKE_FILES)
+      file(MAKE_DIRECTORY \"\${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_DATADIR}/cmake/spdlog\")
+      file(COPY \${SPDLOG_CMAKE_FILES}
+        DESTINATION \"\${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_DATADIR}/cmake/spdlog\")
+      file(REMOVE_RECURSE \"\${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}/cmake/spdlog\")
+    endif()
+  ")
+endif()

cmake/mqt-core-config.cmake.in

@@ -14,7 +14,7 @@ list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}")
 
 include(CMakeFindDependencyMacro)
 find_dependency(nlohmann_json)
-
+find_dependency(spdlog)
 find_dependency(qdmi)
 
 option(MQT_CORE_WITH_GMP "Library is configured to use GMP" @MQT_CORE_WITH_GMP@)