libtool: trust -print-search-dirs from recent GCC

libtool: trust -print-search-dirs from recent GCC

Alan Modra hints in [1] that -print-search-dirs was fixed in
GCC 4.2(?), so that it nowadays automatically appends
-print-multi-os-directory for the applicable directories. I.e.
it should no longer be necessary for libtool to append a second
../lib64 when GCC has already done so. Also, the multi-os
appending loop seems to have been added specifically for early
(arguably broken) bi-arch enabled GCCs that printed -m32
directories even though -m64 was the default [2]. So, my
conclusion is that we want any libtool magic to affect
-print-search-dirs output from contemporary GCCs as little as
possible, while continuing to append the
-print-multi-os-directory for the legacy case.

Fixes bug#15321 reported by Ozkan Sezer.

[1] http://gcc.gnu.org/bugzilla/show_bug.cgi?id=20425
[2] http://lists.gnu.org/archive/html/bug-libtool/2006-09/msg00019.html

* m4/libtool.m4 (_LT_SYS_DYNAMIC_LINKER): If any of the
directories printed by -print-search-dirs ends with the
content of -print-multi-os-directory, then assume that
GCC adds the multi-os-directory where appropriate all by
itself and hence don't try to second guess when to add
it manually.
* THANKS: Update.