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In reply to msg168184, LIBDIR is set to include lib64 instead of lib because openSUSE explicitly does it that way in their multilib implementation.
More specifically, the CONFIG_SITE environment variable is set to /usr/share/site/x86_64-unknown-linux-gnu which contains, among more stuff:
catalin@opensuse:~/hacking/cpython> cat $CONFIG_SITE | grep libdir
# If user did not specify libdir, guess the correct target:
if test "$libdir" = '${exec_prefix}/lib' ; then
 libdir='${exec_prefix}/lib64'
/usr/share/site/x86_64-unknown-linux-gnu is owned by package site-config whose README says:
site-config: Site Paths Configuration for autoconf Based configure Scripts
==========================================================================
Site configuration for autoconf based configure scripts provides smart
defaults for paths that are not specified.
All autoconf based configure scripts will automatically resource site
script using CONFIG_SITE environment variable. It works without any
explicit user interaction.
Currently implemented features:
Automatic libdir setup to $exec_prefix/lib or $exec_prefix/lib64
----------------------------------------------------------------
Depending on architecture, site script should correctly and
automatically switch between lib and lib64 libdir.
libexecdir setup to $exec_prefix/lib
------------------------------------
Upstream libexecdir defaults to $exec_prefix/libexec. This directory is
not part of FHS 2.2, so we change it to $exec_prefix/lib (yes, it is
correct to set it to $exec_prefix/lib even for bi-arch platforms). Most
projects add package name to this path, so you most probably get what
FHS 2.2 expects.