Use CMAKE_INSTALL_PREFIX as ASSIMP_ROOT_DIR.

Since the ASSIMP_LIB_INSTALL_DIR where this CMake config file is
installed to may contain multiple nested directories, we should not use
it to back-calculate the location of the ASSIMP_ROOT_DIR unless we can
take the directory depth into account. Instead use the
CMAKE_INSTALL_PREFIX as the ASSIMP_ROOT_DIR at configure-time.

This approach is more flexible with respect to install directories but
it means that assimp files are not relocatable after installation and I
don't think it works if a DESTDIR option is provided when running `make
install` after cmake configuration.

assimp-config.cmake.in

@@ -10,10 +10,7 @@
 # ASSIMP_LIBRARY_DIRS - link directories
 # ASSIMP_LIBRARIES - libraries to link plugins with
 # ASSIMP_Boost_VERSION - the boost version assimp was compiled with
-get_filename_component(_PREFIX "${CMAKE_CURRENT_LIST_FILE}" PATH)
-get_filename_component(_PREFIX "${_PREFIX}" PATH)
-get_filename_component(_PREFIX "${_PREFIX}" PATH)
-get_filename_component(ASSIMP_ROOT_DIR "${_PREFIX}" PATH)
+get_filename_component(ASSIMP_ROOT_DIR "@CMAKE_INSTALL_PREFIX@" REALPATH)
 
 if( MSVC )
   # in order to prevent DLL hell, each of the DLLs have to be suffixed with the major version and msvc prefix