Enable LTO for build

[Skylion007]

One option to improve the size / performance of the conda package that would only come at the expense of compile time would be to enable Link Time Optimization for the conda recipe. This could be accomplished by adding -DLLVM_ENABLE_LTO=ON to the build.sh script (the default is off). It is not enabled automatically by building in Release mode. It would have the benefit of both generating faster binaries as well as binaries that are potentially smaller.

[isuruf]

I don't think that's a good idea. LTO implies we need to use the same compiler/compiler version for all downstream packages which we are not going to enforce.