"./configure --help" doesn't mention LIBS

[Olly Betts]

This is with autoconf 2.60.

After mail from a user who'd tried to specify "-l<library>" in LDFLAGS
I was suprised to realise that the output of "./configure --help"
doesn't mention LIBS at all.  I think it should - while the autoconf
info manual documents both LDFLAGS and LIBS and the distinction between
them, end users aren't likely to read this, and it's them who will
actually need to pass LIBS and LDFLAGS - the developer will typically
fix configure to automatically detect required libraries on platforms
they test on.

Digging deeper, I noticed LIBS doesn't seem to get marked as precious,
which I think must be an oversight.

Cheers,
    Olly