Fix check for LZ4::LZ4 target (#364)

Author: fdr400

cmake/FindLZ4.cmake

@@ -17,7 +17,7 @@ find_package_handle_standard_args(LZ4 DEFAULT_MSG LZ4_LIBRARIES LZ4_INCLUDE_DIRS
 
 mark_as_advanced(LZ4_LIBRARIES LZ4_INCLUDE_DIRS)
 
-if(LZ4_FOUND AND NOT LZ4::LZ4)
+if(LZ4_FOUND AND NOT TARGET LZ4::LZ4)
     add_library(LZ4::LZ4 UNKNOWN IMPORTED)
     set_target_properties( LZ4::LZ4 PROPERTIES
         INTERFACE_INCLUDE_DIRECTORIES "${LZ4_INCLUDE_DIRS}"