py: Use <alloca.h> for alloca()

alloca() is declared in alloca.h which als happens to be included by stdlib.h.
On mingw however it resides in malloc.h only.
So if we include alloca.h directly, and add an alloca.h for mingw in it's port
directory we can get rid of the mingw-specific define to include malloc.h
and the other ports are happy as well.
Merge pull request #552 from stinos/mingw-alloca

py: Use <alloca.h> for alloca()