Add quotes to build type variable.

assimp-config.cmake.in

@@ -40,7 +40,7 @@ set( ASSIMP_LINK_FLAGS "" )
 set( ASSIMP_LIBRARY_DIRS "${ASSIMP_ROOT_DIR}/@ASSIMP_LIB_INSTALL_DIR@")
 set( ASSIMP_INCLUDE_DIRS "${ASSIMP_ROOT_DIR}/@ASSIMP_INCLUDE_INSTALL_DIR@")
 set( ASSIMP_LIBRARIES assimp${ASSIMP_LIBRARY_SUFFIX})
-if (${CMAKE_BUILD_TYPE} STREQUAL "Debug")
+if ("${CMAKE_BUILD_TYPE}" STREQUAL "Debug")
   set( ASSIMP_LIBRARIES ${ASSIMP_LIBRARIES}@ASSIMP_DEBUG_POSTFIX@)
 endif ()