Fix the cmake configuration of the external packages

CMakeLists.txt

@@ -34,7 +34,7 @@ add_subdirectory(libs)
 # define an exported dune library
 dune_add_library(dunemeshdist INTERFACE
   EXPORT_NAME MeshDist NAMESPACE Dune::)
-target_compile_features(dunemeshdist INTERFACE CXX_std_20)
+target_compile_features(dunemeshdist INTERFACE cxx_std_20)
 
 # link external packages
 if(ENABLE_NANOFLANN)

libs/CMakeLists.txt

@@ -7,7 +7,10 @@ if(ENABLE_NANOFLANN)
     GIT_REPOSITORY https://github.com/jlblancoc/nanoflann.git   # the repository
     GIT_TAG        v1.6.2                                       # the tag
     )
-  FetchContent_MakeAvailable(nanoflann)
+  FetchContent_GetProperties(nanoflann)
+  if(NOT nanoflann_POPULATED)
+    FetchContent_Populate(nanoflann)
+  endif()
 
   file(GLOB NANOFLANN_HEADERS CONFIGURE_DEPENDS "${nanoflann_SOURCE_DIR}/include/*.hpp")