[ODE] problems building ODE on mingw

[Nicolas Weber]

Hi,

I've built ODE on mingw (gcc 3.2). I had to make some changes because 
alloca() is no longer defined in mingw (because it's not standard?). 
Here are the changes:

ode/common.h:
in macro dALLOCA16:
replace alloca with __builtin_alloca

src/step.cpp lines 439, 727:
replace alloca with ALLOCA

src/collision_space.cpp lines 513, 540:
replace alloca with ALLOCA

Regards,
Nico