Use FetchContent_MakeAvailable for asio and eigen

CMake 3.30 deprecates FetchContent_Populate as CMP0169.  FetchContent_MakeAvailable is already used in tests/CMakeLists.txt so this should not cause any problems for people on older versions of CMake.

Tested:
Built and ran tests using default container with LLVM-19 and CMake 3.30.2 on x86_64 without eigen installed on the image.

tests/asio_repe/CMakeLists.txt

@@ -6,10 +6,7 @@ FetchContent_Declare(
     GIT_TAG asio-1-30-1
     GIT_SHALLOW TRUE
 )
-FetchContent_GetProperties(asio)
-if(NOT asio_POPULATED)
-    FetchContent_Populate(asio)
-endif()
+FetchContent_MakeAvailable(asio)
 
 add_subdirectory(server)
 add_subdirectory(client)

tests/eigen_test/CMakeLists.txt

@@ -10,13 +10,7 @@ if(NOT Eigen3_FOUND)
     GIT_TAG 3.4.0
     GIT_SHALLOW TRUE
   )
-  FetchContent_GetProperties(Eigen3)
-  if(NOT eigen3_POPULATED)
-    FetchContent_Populate(Eigen3)
-    add_library(Eigen3 INTERFACE IMPORTED GLOBAL)
-    add_library(Eigen3::Eigen ALIAS Eigen3)
-    target_include_directories(Eigen3 INTERFACE "${eigen3_SOURCE_DIR}")
-  endif()
+  FetchContent_MakeAvailable(Eigen3)
 endif()
 
 target_link_libraries(${PROJECT_NAME} PRIVATE glz_test_common Eigen3::Eigen)