Avoid configuring Eigen via FetchContent_MakeAvaiable

Eigen's configuration script is really verbose and we don't need its
output when configuring MRtrix3 as it's just a header only library.
This also allows to get rid of the custom patch to fix this problem.

cmake/Dependencies.cmake

@@ -8,18 +8,26 @@ else()
 endif()
 
 # Eigen
+# We avoid configuring the Eigen library via FetchContent_MakeAvaiable
+# to avoid the verbosity of Eigen's CMake configuration output.
 if(MRTRIX_LOCAL_DEPENDENCIES)
     set(eigen_url ${MRTRIX_DEPENDENCIES_DIR}/eigen-3.4.0.tar.gz)
 else()
     set(eigen_url "https://gitlab.com/libeigen/eigen/-/archive/3.4.0/eigen-3.4.0.tar.gz")
 endif()
-FetchContent_MakeAvailable(eigen3)
 
 FetchContent_Declare(
     eigen3
     DOWNLOAD_EXTRACT_TIMESTAMP ON
     URL ${eigen_url}
 )
+FetchContent_GetProperties(Eigen3)
+if(NOT eigen3_POPULATED)
+    FetchContent_Populate(Eigen3)
+    add_library(Eigen3 INTERFACE)
+    add_library(Eigen3::Eigen ALIAS Eigen3)
+    target_include_directories(Eigen3 INTERFACE "${eigen3_SOURCE_DIR}")
+endif()
 
 # Json for Modern C++
 if(MRTRIX_LOCAL_DEPENDENCIES)

cmd/CMakeLists.txt

@@ -9,7 +9,6 @@ if(MRTRIX_USE_PCH)
     file(GENERATE OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/pch_cmd.cpp CONTENT "int main(){}")
     add_executable(pch_cmd ${CMAKE_CURRENT_BINARY_DIR}/pch_cmd.cpp)
     target_include_directories(pch_cmd PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/../core)
-    find_package(Eigen3 REQUIRED)
     target_link_libraries(pch_cmd PRIVATE Eigen3::Eigen half::half)
     target_precompile_headers(pch_cmd PRIVATE
         [["app.h"]]