configure: Add link-time optimization support
Enabled by using the --enable-lto configuration option. May give a slight performance improvement in some cases, but it can also reduce performance in other cases (largely compiler-dependant) so don't enable it by default. It also makes compilation (and linking in particular) a fair bit slower. Note that some older versions of GNU binutils will incorrectly warn about "memset used with constant zero length parameter" when linking using LTO. This is due to a bug in binutils and can safely be ignored.