Allow to call find_package(rerun_sdk) two or more times (#5886)

Fix #5885 .


### What

### Checklist
* [x] I have read and agree to [Contributor
Guide](https://github.com/rerun-io/rerun/blob/main/CONTRIBUTING.md) and
the [Code of
Conduct](https://github.com/rerun-io/rerun/blob/main/CODE_OF_CONDUCT.md)
* [x] I've included a screenshot or gif (if applicable)
* [x] I have tested the web demo (if applicable):
* Using newly built examples:
[rerun.io/viewer](https://rerun.io/viewer/pr/5886)
* Using examples from latest `main` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5886?manifest_url=https://app.rerun.io/version/main/examples_manifest.json)
* Using full set of examples from `nightly` build:
[rerun.io/viewer](https://rerun.io/viewer/pr/5886?manifest_url=https://app.rerun.io/version/nightly/examples_manifest.json)
* [x] The PR title and labels are set such as to maximize their
usefulness for the next release's CHANGELOG
* [x] If applicable, add a new check to the [release
checklist](https://github.com/rerun-io/rerun/blob/main/tests/python/release_checklist)!

- [PR Build Summary](https://build.rerun.io/pr/5886)
- [Recent benchmark results](https://build.rerun.io/graphs/crates.html)
- [Wasm size tracking](https://build.rerun.io/graphs/sizes.html)

rerun_cpp/Config.cmake.in

@@ -5,16 +5,18 @@ include("${CMAKE_CURRENT_LIST_DIR}/rerun_sdkTargets.cmake")
 set(RERUN_LIB_DIR "${CMAKE_CURRENT_LIST_DIR}/../..")
 
 if(@RERUN_INSTALL_RERUN_C@)
-    # Setup `rerun_c` (imported libraries can't be exported!)
-    add_library(rerun_c STATIC IMPORTED GLOBAL)
-    get_filename_component(RERUN_C_LIB_NAME "@RERUN_C_LIB_LOCATION@" NAME)
-    set_target_properties(rerun_c PROPERTIES IMPORTED_LOCATION "${RERUN_LIB_DIR}/${RERUN_C_LIB_NAME}")
-    if(APPLE)
-        target_link_libraries(rerun_c INTERFACE "-framework CoreFoundation" "-framework IOKit")
-    elseif(UNIX) # if(LINUX) # CMake 3.25
-        target_link_libraries(rerun_c INTERFACE "-lm -ldl -pthread")
-    elseif(WIN32)
-        target_link_libraries(rerun_c INTERFACE ws2_32.dll Bcrypt.dll Userenv.dll ntdll.dll)
+    if(NOT TARGET rerun_c)
+        # Setup `rerun_c` (imported libraries can't be exported!)
+        add_library(rerun_c STATIC IMPORTED GLOBAL)
+        get_filename_component(RERUN_C_LIB_NAME "@RERUN_C_LIB_LOCATION@" NAME)
+        set_target_properties(rerun_c PROPERTIES IMPORTED_LOCATION "${RERUN_LIB_DIR}/${RERUN_C_LIB_NAME}")
+        if(APPLE)
+            target_link_libraries(rerun_c INTERFACE "-framework CoreFoundation" "-framework IOKit")
+        elseif(UNIX) # if(LINUX) # CMake 3.25
+            target_link_libraries(rerun_c INTERFACE "-lm -ldl -pthread")
+        elseif(WIN32)
+            target_link_libraries(rerun_c INTERFACE ws2_32.dll Bcrypt.dll Userenv.dll ntdll.dll)
+        endif()
     endif()
 endif()
 
@@ -23,15 +25,19 @@ endif()
 if(@RERUN_DOWNLOAD_AND_BUILD_ARROW@ AND NOT @RERUN_ARROW_LINK_SHARED@)
     message(STATUS "Rerun is using bundled arrow library.")
 
-    add_library(rerun_arrow_target STATIC IMPORTED GLOBAL)
-    get_filename_component(RERUN_ARROW_LIB_NAME "@RERUN_ARROW_LIBRARY_FILE@" NAME)
-    set_target_properties(rerun_arrow_target PROPERTIES IMPORTED_LOCATION "${RERUN_LIB_DIR}/${RERUN_ARROW_LIB_NAME}")
+    if(NOT TARGET rerun_arrow_target)
+        add_library(rerun_arrow_target STATIC IMPORTED GLOBAL)
+        get_filename_component(RERUN_ARROW_LIB_NAME "@RERUN_ARROW_LIBRARY_FILE@" NAME)
+        set_target_properties(rerun_arrow_target PROPERTIES IMPORTED_LOCATION "${RERUN_LIB_DIR}/${RERUN_ARROW_LIB_NAME}")
+    endif()
 
     # We have to explicitly opt in the arrow bundled dependencies, otherwise we're missing the symbols for mimalloc.
-    add_library(arrow_targetBundledDeps STATIC IMPORTED)
-    get_filename_component(RERUN_ARROW_DEPS_LIB_NAME "@RERUN_ARROW_BUNDLED_DEPENDENCIES_FILE@" NAME)
-    set_target_properties(arrow_targetBundledDeps PROPERTIES IMPORTED_LOCATION "${RERUN_LIB_DIR}/${RERUN_ARROW_DEPS_LIB_NAME}")
-    target_link_libraries(rerun_arrow_target INTERFACE arrow_targetBundledDeps)
+    if(NOT TARGET arrow_targetBundledDeps)
+        add_library(arrow_targetBundledDeps STATIC IMPORTED)
+        get_filename_component(RERUN_ARROW_DEPS_LIB_NAME "@RERUN_ARROW_BUNDLED_DEPENDENCIES_FILE@" NAME)
+        set_target_properties(arrow_targetBundledDeps PROPERTIES IMPORTED_LOCATION "${RERUN_LIB_DIR}/${RERUN_ARROW_DEPS_LIB_NAME}")
+        target_link_libraries(rerun_arrow_target INTERFACE arrow_targetBundledDeps)
+    endif()
 else()
     if(@RERUN_DOWNLOAD_AND_BUILD_ARROW@ AND @RERUN_ARROW_LINK_SHARED@)
         message(WARNING
@@ -45,9 +51,11 @@ else()
 
     message(STATUS "Rerun is using a system installed libArrow.")
 
-    if(@RERUN_ARROW_LINK_SHARED@)
-        add_library(rerun_arrow_target ALIAS Arrow::arrow_shared)
-    else()
-        add_library(rerun_arrow_target ALIAS Arrow::arrow_static)
+    if(NOT TARGET rerun_arrow_target)
+        if(@RERUN_ARROW_LINK_SHARED@)
+            add_library(rerun_arrow_target ALIAS Arrow::arrow_shared)
+        else()
+            add_library(rerun_arrow_target ALIAS Arrow::arrow_static)
+        endif()
     endif()
 endif()