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.