Use native compilation for allocators

[jvoisin]

As discussed today, there are pros and cons about using native compilation for allocators (like in #54), but mostly pros, boiling down to allocators taking advantage of modern CPU features shouldn't be penalized by using the lowest common denominator.

Since not every build system/scripts used have a --enable-native-and-all-perf-stuff-go-go-go option, one way to make use of --march-native would be to simply pass it via the CFLAGS and CXXFLAGS environment variables.