Fix CMake import

Using CMake 3.15 & Visual Studio 2019 to build on windows produced a cmake configuration which was unable to be used by clients.  Investigating this I was eventually able to determine this was caused by a mismatch in the target properties.  Specifically, it was setting the TYPE property to STATIC_LIBRARY even though it was building as a SHARED_LIBRARY.  

This in turn was because the if clause on line `if(@BUILD_SHARED_LIBS@)` of this file was evaluating to `if (ON)` which was then not properly evaluated while under the CMake 2.6 policy.  Explicitly setting the `CMP0012` policy to NEW fixes the problem.

assimpTargets.cmake.in

@@ -5,6 +5,9 @@ if("${CMAKE_MAJOR_VERSION}.${CMAKE_MINOR_VERSION}" LESS 2.5)
 endif()
 cmake_policy(PUSH)
 cmake_policy(VERSION 2.6)
+# Required for the evaluation of "if(@BUILD_SHARED_LIBS@)" below to function
+cmake_policy(SET CMP0012 NEW)
+
 #----------------------------------------------------------------
 # Generated CMake target import file.
 #----------------------------------------------------------------