Unvendor Eigen and bump to 3.4.1

CMakeLists.txt

@@ -27,7 +27,7 @@
 # License along with Mutation++.  If not, see
 # <http://www.gnu.org/licenses/>.
 #
-cmake_minimum_required(VERSION 3.10...4.0)
+cmake_minimum_required(VERSION 3.15...4.0)
 cmake_policy(SET CMP0048 NEW)
 set(CMAKE_CXX_STANDARD 14)
 
@@ -204,10 +204,18 @@ endif()
 ###############################################################################
 # ThirdParty Libraries
 ###############################################################################
+include(FetchContent)
 
 # Eigen
-find_package(Eigen3)
-
+FetchContent_Declare(
+  Eigen3
+  GIT_REPOSITORY https://gitlab.com/libeigen/eigen.git
+  GIT_TAG        3.4.1
+  GIT_SHALLOW    TRUE
+  FIND_PACKAGE_ARGS
+)
+set(EIGEN_BUILD_CMAKE_PACKAGE YES)
+FetchContent_MakeAvailable(Eigen3)
 
 ###############################################################################
 # Source code

docs/dependencies.md

@@ -9,7 +9,6 @@ If you don't have admin rights on your machine and you cannot get a module, we p
 
 The source code of third-party dependencies is located in the `thirdparty` directory. The code for the following dependencies is taken directly from a git repository using the [subrepo](https://github.com/ingydotnet/git-subrepo) utility:
 
-- Eigen ([GitHub mirror of the BitBucket Mercurial repository](https://github.com/RLovelett/eigen))
 - Catch ([original GitHub project page](https://github.com/philsquared/Catch))
 
 The installation process of the `subrepo` utility is detailed on the project page.
@@ -28,12 +27,6 @@ If a dependency is managed using `subrepo`, it can be updated by using it. From
 git subrepo checkout <path_to_dependency> -b <tag_or_branch>
 ```
 
-For instance, to upgrade Eigen to v3.3.2:
-
-```bash
-git subrepo checkout thirdparty/eigen -b 3.3.2
-```
-
 Check on the hosting git repo the tag or branch name to use.
 
 If the code is not managed using `subrepo`, delete the current code and replace it with the new version.