Nested Superbuild Support

Dependent projects might have the same dependencies and build
as a superbuild as well.

CMakeLists.txt

@@ -255,7 +255,9 @@ endif()
 
 
 # external library: nlohmann-json (required)
-if(openPMD_USE_INTERNAL_JSON)
+if(TARGET nlohmann_json::nlohmann_json)
+    # nothing to do, target already exists in the superbuild
+elseif(openPMD_USE_INTERNAL_JSON)
     set(JSON_BuildTests OFF CACHE INTERNAL "NLohmann JSON option defiend internally by openPMD")
     set(JSON_Install OFF CACHE INTERNAL "NLohmann JSON option defiend internally by openPMD")  # only used PRIVATE
 
@@ -285,7 +287,9 @@ target_link_libraries(openPMD::thirdparty::nlohmann_json
     INTERFACE nlohmann_json::nlohmann_json)
 
 # external library: toml11
-if(openPMD_USE_INTERNAL_TOML11)
+if(TARGET toml11::toml11)
+    # nothing to do, target already exists in the superbuild
+elseif(openPMD_USE_INTERNAL_TOML11)
     set(toml11_INSTALL OFF CACHE INTERNAL "toml11 option defined internally by openPMD")
 
     # Git fetcher
@@ -440,7 +444,9 @@ else()
     set(openPMD_HAVE_PYTHON FALSE)
 endif()
 if(Python_FOUND)
-    if(openPMD_USE_INTERNAL_PYBIND11)
+    if(TARGET pybind11::module)
+        # nothing to do, target already exists in the superbuild
+    elseif(openPMD_USE_INTERNAL_PYBIND11)
         # Git fetcher
         set(openPMD_pybind11_repo "https://github.com/pybind/pybind11.git"
             CACHE STRING
@@ -589,7 +595,9 @@ target_include_directories(openPMD PUBLIC
 
 # Catch2 for unit tests
 if(openPMD_BUILD_TESTING)
-    if(openPMD_USE_INTERNAL_CATCH)
+    if(TARGET Catch2::Catch2)
+        # nothing to do, target already exists in the superbuild
+    elseif(openPMD_USE_INTERNAL_CATCH)
         # Git fetcher
         set(openPMD_catch_repo "https://github.com/catchorg/Catch2.git"
             CACHE STRING