[Dune] Back-porting autoconf warnings and SuperLU 4.3 support

[Christoph Grüninger]

Hi Dune,
does someone object if I back-port the following changes from trunk to 
2.1.2-svn?

- dune-common r6499, r6498, r6497, r6496
   "use proper quoting to avoid autoconf warning on missing
   AC_LANG_SOURCE"
   These fix lengthy and annoying warnings in autoconf.

- dune-istl r1527, r1529
   "Added support for SuperLU 4.3. We are affected by the changed symbol
    "DOUBLE" to "SLU_DOUBLE".
   Deprecated pre-2005 support of SuperLU."
   Excluding the deprecation of pre 2005 versions.

Or is a Dune 2.2 release planned for the near future? Either we 
stabilize the current trunk and release before the developer meeting or 
after the meeting new feature will be added, than we should provide 
2.1.x bugfix releases.

Bye
Christoph

-- 
Unix is simple and coherent, but it takes a genius – or at any
rate a programmer – to understand and appreciate the simplicity.
[Dennis Ritchie, 1941-2011, created C and Unix]