Simplify creating the library path

Instead of checking whether the library path ends up somewhere in /usr,
set the architecture prefix, anyway. The library path mechanism is also
used in custom prefixes, without this change, the library gets installed
globally on Debian and Ubuntu, leading to non-standard behaviour and
cross-build problems, as multiple architecture builds of this library
can't be installed alongside each other.

This is the minimal change to correct this behaviour from upstream cmake.

cmake/modules/GNUInstallDirs.cmake

@@ -134,13 +134,7 @@ 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()
+        set(_LIBDIR_DEFAULT "lib/${CMAKE_LIBRARY_ARCHITECTURE}")
       endif()
     else() # not debian, rely on CMAKE_SIZEOF_VOID_P:
       if(NOT DEFINED CMAKE_SIZEOF_VOID_P)