Support GCC LTO on GNU/Linux.

Support GCC LTO on GNU/Linux.

* libltdl/config/ltmain.m4sh (func_mode_link): Allow through
flags matching -O*, -flto*, -fwhopr, -fuse-linker-plugin.
* libltdl/m4/libtool.m4 (_LT_CMD_GLOBAL_SYMBOLS): Drop symbols
starting with __gnu_lto.
(_LT_LINKER_SHLIBS) [linux] <archive_cmds, archive_expsyms_cmds>:
Add $pic_flag for GCC.
(_LT_LANG_CXX_CONFIG) [linux] <archive_cmds, archive_expsyms_cmds>:
Likewise.
(_LT_SYS_HIDDEN_LIBDEPS): Ignore files matching *.lto.o.
* NEWS: Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>