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.
The one specified by mdoc is hard to read for non-native
speakers from countries which read the date day-first (like
Germany, Greece, North-Korea, Swamp,...).
This is also consistent with how we generally specify dates
at suckless.org.
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.
