Add named-lto option to meson build to named with LTO

Add named-lto option to meson build to named with LTO

Enabling LTO yields substantial performance gains on both authoritative
and resolver benchmarks.
But since LTO defers many optimization passes to link time, enabling LTO
across the board would cause an increase in compilation time, as passes
that would be run only once would need to be run for each executable.

As a compromise, this commit adds a named-lto build option, that
compiles the individual object files with the -ffat-lto-object option
and then enables LTO only for the named executable. Object files are
reused between lib*.so and the named executable.