ISO C does not require that main() have an explicit return value
(if the end of the main function is reached, it is as if it returned
0). However, when built as sbase-box, the main function is turned
into getconf_main, which does not have the same special treatment.
Printing `undefined` to stdout means that the variable is valid but not
set. Instead, report an error in this case.
linux 4.13 appends the result of `getconf LFS_CFLAGS` to HOSTCFLAGS,
even if it prints `undefined`. This is arguably a bug in the Makefile,
but even so, getconf shouldn't report that unsupported variables are
The logic is simple, it's just a pain in the ass to fill the
Some lines had to be commented out, as glibc/musl apparently
have not fully implemented the mandatory variables for the
2013 corrigendum of POSIX 2008.
Also added a manpage and the necessary entries in README.
I also removed it from the TODO.