Improve description of the various *LINK variables.

Improve description of the various *LINK variables.

* doc/automake.texi (Program and Library Variables): _LINK also
receives libraries to link against.  _LINK may be generated.
(Program Variables): Document reasons when per-target _LINK is
used instead of LINK.
(How the Linker is Chosen): Document how a per-target _LINK
variable and per-target link flags override linker selection.
* THANKS: Update.
Report by Dave Korn against gcc/libstdc++-v3.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>