Check for variable definition rather than empty string

CMakeLists.txt

@@ -342,19 +342,19 @@ INCLUDE (PrecompiledHeader)
 
 # Set Assimp project output directory variables.
 # Will respect top-level CMAKE_*_OUTPUT_DIRECTORY variables if any are set.
-IF(CMAKE_RUNTIME_OUTPUT_DIRECTORY STREQUAL "")
+IF(NOT DEFINED CMAKE_RUNTIME_OUTPUT_DIRECTORY)
   SET(ASSIMP_RUNTIME_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/bin" CACHE STRING "Path for runtime output files")
 ELSE()
   SET(ASSIMP_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_RUNTIME_OUTPUT_DIRECTORY} CACHE STRING "Path for runtime output files")
 ENDIF()
 
-IF(CMAKE_LIBRARY_OUTPUT_DIRECTORY STREQUAL "")
+IF(NOT DEFINED CMAKE_LIBRARY_OUTPUT_DIRECTORY)
   SET(ASSIMP_LIBRARY_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/bin" CACHE STRING "Path for library output files")
 ELSE()
   SET(ASSIMP_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_LIBRARY_OUTPUT_DIRECTORY} CACHE STRING "Path for runtime output files")
 ENDIF()
 
-IF(CMAKE_ARCHIVE_OUTPUT_DIRECTORY STREQUAL "")
+IF(NOT DEFINED CMAKE_ARCHIVE_OUTPUT_DIRECTORY)
   SET(ASSIMP_ARCHIVE_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/lib" CACHE STRING "Path for library output files")
 ELSE()
   SET(ASSIMP_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_ARCHIVE_OUTPUT_DIRECTORY} CACHE STRING "Path for runtime output files")