Generate /usr/libexec/getconf files when cross-compiling

[Roland McGrath]

> The sysconf and confstr cases want to distinguish between three cases 
> (undefined; defined and positive; defined and not positive), where the 
> build-time case only wants to distinguish the defined-and-positive case 
> from the rest.  The different patterns for the names for V5 from those for 
> V6 and V7 (_XBS5_ILP32_OFF32 and _SC_XBS5_ILP32_OFF32; 
> _POSIX_V7_ILP32_OFF32 and _SC_V7_ILP32_OFF32) also mean you can't just use 
> single-argument macros like that.

Well, fine.  So the exact way to define and use the macros is a bit
different from my strawman example.  None of that seems like it would
make the scheme any less workable.