cmake: fix install dir if CMAKE_INSTALL_PREFIX is set

CMakeLists.txt

@@ -64,8 +64,7 @@ set(CMAKE_VISIBILITY_INLINES_HIDDEN ON)
 set(CMAKE_INCLUDE_CURRENT_DIR ON) # for .moc
 # Instruct CMake to run moc automatically when needed.
 set(CMAKE_AUTOMOC ON)
-
-if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
+if(NOT CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
   set(QTAV_INSTALL_HEADERS ${CMAKE_INSTALL_PREFIX}/include)
   set(QTAV_INSTALL_LIBS ${CMAKE_INSTALL_PREFIX}/lib)
   set(QTAV_INSTALL_BINS ${CMAKE_INSTALL_PREFIX}/bin)
@@ -93,7 +92,7 @@ set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/lib)
 
 install(FILES
   cmake/FindQtAV.cmake
-  DESTINATION ${QT_INSTALL_LIBS}/cmake
+  DESTINATION ${QTAV_INSTALL_LIBS}/cmake
 )
 
 if(CMAKE_GENERATOR MATCHES "Visual Studio.*")