cmake: only find_package(OVM) if the target does not exist

examples/CMakeLists.txt

@@ -2,7 +2,9 @@ cmake_minimum_required(VERSION 3.12 FATAL_ERROR)
 cmake_policy(VERSION 3.12.0)
 project(OpenVolumeMeshExamples)
 
-find_package(OpenVolumeMesh REQUIRED)
+if (NOT TARGET OpenVolumeMesh::OpenVolumeMesh)
+    find_package(OpenVolumeMesh REQUIRED)
+endif()
 
 # Add target for first example
 add_executable(simple_mesh simple_mesh/simple_mesh.cc)

src/Unittests/CMakeLists.txt

@@ -7,7 +7,9 @@ SET(SOURCE_FILES
     unittests_iterators.cc
     unittests_properties.cc)
 
-find_package(OpenVolumeMesh REQUIRED)
+if (NOT TARGET OpenVolumeMesh::OpenVolumeMesh)
+    find_package(OpenVolumeMesh REQUIRED)
+endif()
 
 set(TARGET_NAME "${OVM_TARGET_PREFIX}unittests")