assimpTargets.cmake: define add_library() with lib type directly

Instead of using if(ON)/if(OFF) to determine which of the "hardcoded"
add_library(... SHARED ...)  or (... STATIC ...) should be used,
specify a new BUILD_LIB_TYPE variable that is set directly to either
SHARED or STATIC and substituted in the `add_library()` statement
when assimpTargets.cmake.in is configured.

This removes a CMP0012 collision with prior `cmake_policy(VERSION 2.6)`
statement and makes the CMP0012 warning not appear in users' projects.
This problem is mentioned in issue #2685.

CMakeLists.txt

@@ -391,6 +391,11 @@ IF(HUNTER_ENABLED)
   )
 ELSE(HUNTER_ENABLED)
   # cmake configuration files
+  if(${BUILD_SHARED_LIBS})
+    set(BUILD_LIB_TYPE SHARED)
+  else()
+    set(BUILD_LIB_TYPE STATIC)
+  endif()
   CONFIGURE_FILE("${CMAKE_CURRENT_SOURCE_DIR}/assimp-config.cmake.in"         "${CMAKE_CURRENT_BINARY_DIR}/assimp-config.cmake" @ONLY IMMEDIATE)
   CONFIGURE_FILE("${CMAKE_CURRENT_SOURCE_DIR}/assimpTargets.cmake.in"         "${CMAKE_CURRENT_BINARY_DIR}/assimpTargets.cmake" @ONLY IMMEDIATE)
   IF (is_multi_config)

assimpTargets.cmake.in

@@ -51,11 +51,7 @@ if(_IMPORT_PREFIX STREQUAL "/")
 endif()
 
 # Create imported target assimp::assimp
-if(@BUILD_SHARED_LIBS@)
-  add_library(assimp::assimp SHARED IMPORTED)
-else()
-  add_library(assimp::assimp STATIC IMPORTED)
-endif()
+add_library(assimp::assimp @BUILD_LIB_TYPE@ IMPORTED)
 
 set_target_properties(assimp::assimp PROPERTIES
   COMPATIBLE_INTERFACE_STRING "assimp_MAJOR_VERSION"