Merge pull request #3899 from sebasje/sebas/cmake

fix multiarch library path for Debian/Ubuntu

cmake/modules/GNUInstallDirs.cmake

@@ -134,14 +134,8 @@ if(NOT DEFINED CMAKE_INSTALL_LIBDIR OR (_libdir_set
       AND NOT CMAKE_CROSSCOMPILING)
     if (EXISTS "/etc/debian_version") # is this a debian system ?
       if(CMAKE_LIBRARY_ARCHITECTURE)
-        if("${CMAKE_INSTALL_PREFIX}" MATCHES "^/usr/?$")
         set(_LIBDIR_DEFAULT "lib/${CMAKE_LIBRARY_ARCHITECTURE}")
       endif()
-        if(DEFINED _GNUInstallDirs_LAST_CMAKE_INSTALL_PREFIX
-            AND "${_GNUInstallDirs_LAST_CMAKE_INSTALL_PREFIX}" MATCHES "^/usr/?$")
-          set(__LAST_LIBDIR_DEFAULT "lib/${CMAKE_LIBRARY_ARCHITECTURE}")
-        endif()
-      endif()
     else() # not debian, rely on CMAKE_SIZEOF_VOID_P:
       if(NOT DEFINED CMAKE_SIZEOF_VOID_P)
         message(AUTHOR_WARNING