* Essentially copy and adapt the definition of AC_CHECK_LIB.  This should
   make sure the two macros behave the same even in corner cases (except for
   the added functionality, of course).

 * Properly document what exactly is allowed in the arguments.

Parse arguments to DUNE_CHECK_LIBS_EXT properly in parmetis.  This avoids
expansions like 'LDFLAGS="$LDFLAGS -L"$PARMETISLIBPATH""'.

[[Imported from SVN: r5830]]