doc: fix texinfo macro usage

doc: fix texinfo macro usage

The texinfo manual recommends avoiding the use of a trailing @c in
any macro designed to be used inline (as is the case with our ovar
and dvar macros).  Furthermore, passing '@\n' in the middle of a
macro call is much different than passing '@\n' between arguments
of a @defmac for line continuation.

* doc/autoconf.texi (ovar, dvar): Don't end macro with @c, since
these macros are designed to be embedded in one-line usage.
(Fortran Compiler): Don't split @dvar.
* THANKS: Update.
Reported by Stefano Lattarini.

Signed-off-by: Eric Blake <eblake@redhat.com>
Copyright-paperwork-exempt: Yes