Adding a "release" setting in build.mk.sample (and some other build system questions)

[Harry .]

> https://ghc.haskell.org/trac/ghc/wiki/MakingReleases#Makingthebinarybuilds
> https://ghc.haskell.org/trac/ghc/wiki/Building/Using#Buildconfiguration

So the mysterious SRC_HC_OPTS = -O -H64m which appears in every build flavour isn't used for the release build?