COMP: Allow install lib directory name changes

Follows InsightSoftwareConsortium/ITK@5b09d54

CMakeLists.txt

@@ -39,20 +39,23 @@ set(RTK_LIBRARIES RTK)
 #=========================================================
 # Installation variables
 #=========================================================
+if(NOT CMAKE_INSTALL_LIBDIR)
+  set(CMAKE_INSTALL_LIBDIR lib)
+endif()
 if(NOT RTK_INSTALL_RUNTIME_DIR)
   set(RTK_INSTALL_RUNTIME_DIR bin)
 endif()
 if(NOT RTK_INSTALL_LIB_DIR)
-  set(RTK_INSTALL_LIB_DIR lib)
+  set(RTK_INSTALL_LIB_DIR ${CMAKE_INSTALL_LIBDIR})
 endif()
 if(NOT RTK_INSTALL_ARCHIVE_DIR)
-  set(RTK_INSTALL_ARCHIVE_DIR lib)
+  set(RTK_INSTALL_ARCHIVE_DIR ${CMAKE_INSTALL_LIBDIR})
 endif()
 if(NOT RTK_INSTALL_INCLUDE_DIR)
   set(RTK_INSTALL_INCLUDE_DIR include/RTK)
 endif()
 if(NOT RTK_INSTALL_PACKAGE_DIR)
-  set(RTK_INSTALL_PACKAGE_DIR "lib/cmake/RTK")
+  set(RTK_INSTALL_PACKAGE_DIR "${CMAKE_INSTALL_LIBDIR}/cmake/RTK")
 endif()
 
 #Set position independent code for Unix (-fPIC)
@@ -131,10 +134,10 @@ if(NOT CMAKE_RUNTIME_OUTPUT_DIRECTORY)
   set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${RTK_BINARY_DIR}/bin)
 endif()
 if(NOT CMAKE_LIBRARY_OUTPUT_DIRECTORY)
-  set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${RTK_BINARY_DIR}/lib)
+  set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${RTK_BINARY_DIR}/${CMAKE_INSTALL_LIBDIR})
 endif()
 if(NOT CMAKE_ARCHIVE_OUTPUT_DIRECTORY)
-  set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${RTK_BINARY_DIR}/lib)
+  set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${RTK_BINARY_DIR}/${CMAKE_INSTALL_LIBDIR})
 endif()
 
 #=========================================================