Support Mathematica 9 MathLink directory location on Mac OS

[Barry Wardell]

Mathematica 9 requires

MLINKDIR ?= /Applications/Mathematica.app/SystemFiles/Links/MathLink/DeveloperKit/MacOSX-x86-64/CompilerAdditions 

instead of

MLINKDIR ?= /Applications/Mathematica.app/SystemFiles/Links/MathLink/DeveloperKit/CompilerAdditions

in the Makefile. We should use whichever of these exists.

[Ian Hinder]

This sort of logic is difficult to achieve in a Makefile. Should we be thinking about using autoconf or similar? That also has the advantage that it is more familiar to people than editing the makefile fragment.

[Barry Wardell]

This was fixed in 20df41bbc2f0f142452ce6f4365ff9edc0550073. The fix was to use the path required by Mathematica 9 and 10, so compiling with earlier versions requires MLINKDIR to be set in make.defs.