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
valid.
Every system is going to have a different configuration
so the only solution is to put an ifdef guard for every
value. To do this, we generate the header at compile time
with a shell script.
The logic is simple, it's just a pain in the ass to fill the
data-structures.
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.
