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.
