This commit makes it possible to build the OCaml compiler according to
its configuration by simply runnning make. There is no need to specify
neither world nor world.opt explicitly, although the two targets
remain available.
This commit also introduces (and starts making use of) the
NATIVE_COMPILER build variable whosse value is true when the native
compiler is enabled and false otherwise.
Florian Angeletti
Damien Doligez
Hannes Mehnert
Sébastien Hinderer
Gabriel Scherer