Add --enable-debug and --enable-optimize flag for controlling building debug and optimized verisons of libprocess

Reworks buiding mesos in a "debug" vs. a "release" configuration. By default, mesos is built in a developer-centric setup (No optimizations, minimal debug info), in order to maximize developer productivity

None: '-O0 -g1'
--enable-optimize == '-O2'
--enable-debug == '-g'
--enable-optimize --enable-debug == '-O2 -g'

If a user / developer passes CXXFLAGS or CFLAGS manually, then they are not changed / touched at all. This is important so that Mesos is a good citizen when being built for various distributions (As well as making it so specialized one-off groupings of flags are feasible to use).

Adds two defines for accessing what mode things are being built in: 'DEBUG' and 'OPTIMIZE' which can be hooked into later to enable extra logging and the like. For release builds we may want to set 'NDEBUG' which removes assert()'s, but that is a seperate discussion.

Review: https://reviews.apache.org/r/27252