[patch by Adrien Nader] build: avoid an unneeded gnu-style conditional in Makefile.

The OpenBSD Jenkins slave started failing because of the use of
gnu-style conditionals.
There's unfortunately no way that I know of to get a conditional syntax
that works both with gmake and mk.
Fortunately, the conditional wasn't needed at all since the value was
already computed in the configure step.


git-svn-id: http://caml.inria.fr/svn/ocaml/trunk@13861 f963ae5c-01c2-4b8c-9fe0-0dff7051ff02