Do not re-define targets if find_package(zenohcxx) is called twice (#232

)

install/PackageConfig.cmake.in

@@ -23,13 +23,13 @@ if(_IMPORT_PREFIX STREQUAL "/")
   set(_IMPORT_PREFIX "")
 endif()
 
-set(_ZENOHCXX_BACKEND_FOUND FALSE)
-add_library(zenohcxx INTERFACE IMPORTED)
-target_include_directories(zenohcxx INTERFACE "${_IMPORT_PREFIX}/@CMAKE_INSTALL_INCLUDEDIR@")
+if(NOT TARGET zenohcxx)
+	add_library(zenohcxx INTERFACE IMPORTED)
+	target_include_directories(zenohcxx INTERFACE "${_IMPORT_PREFIX}/@CMAKE_INSTALL_INCLUDEDIR@")
+endif()
 
 # zenohcxx for zenohpico
-if(TARGET zenohpico::lib)
-	set(_ZENOHCXX_BACKEND_FOUND TRUE)
+if(TARGET zenohpico::lib AND NOT TARGET zenohcxx_zenohpico)
 	message(STATUS "defined lib target zenohcxx::zenohpico for zenohpico::lib")
 	add_library(zenohcxx_zenohpico INTERFACE IMPORTED)
 	target_compile_definitions(zenohcxx_zenohpico INTERFACE ZENOHCXX_ZENOHPICO)
@@ -40,8 +40,7 @@ if(TARGET zenohpico::lib)
 endif()
 
 # zenohcxx for zenohc static/dynamic depending on ZENOHC_LIB_STATIC
-if(TARGET zenohc::lib)
-	set(_ZENOHCXX_BACKEND_FOUND TRUE)
+if(TARGET zenohc::lib AND NOT TARGET zenohcxx_zenohc)
 	message(STATUS "defined lib target zenohcxx::zenohc for zenohc::lib")
 	add_library(zenohcxx_zenohc INTERFACE IMPORTED)
 	target_compile_definitions(zenohcxx_zenohc INTERFACE ZENOHCXX_ZENOHC)
@@ -51,6 +50,6 @@ if(TARGET zenohc::lib)
 	add_library(zenohcxx::zenohc ALIAS zenohcxx_zenohc)
 endif()
 
-if(NOT _ZENOHCXX_BACKEND_FOUND)
+if(NOT TARGET zenohcxx_zenohpico AND NOT TARGET zenohcxx_zenohc)
 	message(FATAL_ERROR "Failed to detect zenoh-cpp backend, you need to have either zenoh-c or zenoh-pico installed" )
-endif()
+endif()