Better environ handling w/ MacOS X

Hi Felix,

Here is a patch for posix.scm
103c103,106
< #ifndef C_MACOSX
---
> #ifdef C_MACOSX
> #include <crt_externs.h>
> static char **environ;
> #else
185,189c188
< #ifdef C_MACOSX
< # define C_getenventry(i)   NULL
< #else
< # define C_getenventry(i)   environ[ i ]
< #endif
---
> #define C_getenventry(i)   environ[ i ]
1330a1330,1337
>               (foreign-value #<<EOF
>
> #             ifdef C_MACOSX
>                       (environ = *_NSGetEnviron()),
> #             endif
>                       0
> EOF
>               void)

I think I am abusing 'foreign-value' but seemed the lowest overhead.  
With this method -flat_namespace is not rqrd.

Could configure on AC_CHECK_HEADERS(crt_externs.h) &
AC_CHECK_FUNCS(_NSGetEnviron) but Q&D seems fine for now.

Best Wishes,
Kon